Q: What is the math behind the Colour Wheel I want to create a pie with 12 slices, with each slice a different colour.
Pretty much every colour wheel seems to follow the same format; eg: http://www.tigercolor.com/color-lab/color-theory/color-theory-intro.htm .
But what algorithms are there for generating the colours? What is the math behind RGB(theta)?   Surely there must be some established science on this, but Google is not giving me any clues.

A: A colour wheel (such as the Mac OS X colour picker, pictured below) displays hue and saturation (two of the three components from the HSV colour space). The hue varies with the angle, and the saturation varies with the radius. Typically there's a separate slider for the value (aka brightness).

See Wikipedia for how to convert back and forth between HSV and RGB. Or there may be an API for your programming language of choice. For example, Python has the colorsys library.

A: I was playing around with making an HSL-based color wheel in Excel VBA, using tiny cells as "pixels", and it turned out pretty well, so I thought I'd share.

This demonstrates conversion between HSL and RGB as well as how to programmatically draw lines/circles on any grid — even spreadsheet cells.
The code is ready to run as-is:
Option Explicit

Const colorSheetName = "COLORS"
Const pi = 3.14159265358979
Const squareSize = 3.75             'cell square size (pts)
Const cDiameter = 80#               'circle diameter (cells)
Const numAngles = 360#              'number of angles (lines to draw)

Sub CalculateColorWheel()
  Dim ws As Worksheet, radsPerAngle As Double, radius As Long, xStop As Double, _
      yStop As Double, z As Integer, xyLength As Double, lineDot As Long, _ 
      lineLength As Long, h As Byte, s As Byte, v As Byte, r As Byte, g As Byte, b As Byte

    Set ws = ThisWorkbook.Sheets.Add                        'create new worksheet
    On Error Resume Next                                    'ignore error 
    Application.DisplayAlerts = False                       'ignore warning
    ThisWorkbook.Sheets(colorSheetName).Delete              'delete worksheet (if exists)
    Application.DisplayAlerts = True                        'stop ignoring warnings
    On Error GoTo 0                                         'stop ignoring errors

    With ws
        .Name = colorSheetName                              'name the new sheet
        .Rows.RowHeight = squareSize                        'set rowheight 
        .Columns.ColumnWidth=widthToColumnWidth(squareSize) 'match columnwidth to row
        ActiveWindow.DisplayGridlines = False               'hide gridlines
        ActiveWindow.DisplayHeadings = False                'hide row/col headings
        radius = cDiameter / 2                              'centre point
        lineLength = radius * 1.5                           'dots per angle (line)
        radsPerAngle = (360 / numAngles) * pi / 180         'radians=a(degrees)×pi÷180°

        Debug.Print "Grid size=" & .[a1].Height & "×" & .[a1].Width _
                & ", Diameter:" & cDiameter _
                & ", Area=" & Round(pi * radius ^ 2, 0) _
                & ", Circumference=" & Round(2 * pi * radius, 0) _
                & ", Radians per Angle=" & Round(radsPerAngle, 3) _
                & " × " & numAngles & " angles"                 'stats

        For z = 0 To numAngles - 1  'loop through each angle
            For lineDot = 1 To lineLength   'loop thru length of line
                xyLength = radius * (lineDot / lineLength)  'calc dot xy& offset top-left
                xStop = Int(Cos(radsPerAngle * z) * xyLength) + radius + 2 'x (column)
                yStop = Int(Sin(radsPerAngle * z) * xyLength) + radius + 2 'y (row)

                If .Cells(yStop, xStop).Interior.Pattern=xlNone Then 'skip colored cells
                    h = ((z + 1) / numAngles) * 255                'hue=angle
                    s = (lineDot / lineLength) * 255                    'saturation=radius
                    v = 255                    'maximum brightness. (Adjustable)
                    HSVtoRGB h, s, v, r, g, b                      'convert HSV to RGB
                    .Cells(yStop, xStop).Interior.Color=rgb(r,g,b) 'color the cell
                    dots = dots + 1
                End If
            Next lineDot
            Application.StatusBar = Format(z / (numAngles - 1), "0%") 
            DoEvents    'don't lag
        Next z
    End With
    Beep
    Application.StatusBar = "Finished drawing color circle (" & dots & " colors)"
End Sub

Public Function widthToColumnWidth(pts As Double) As Double
'convert desired column width (points) to Excel "ColWidthUnits"
'12pts and under is a 1:12 ratio of (colWidthUnits:Pts).
' Over 12pts: 1:12 for 1st unit, then 1:(75/11) for remainder
    Select Case pts
        Case Is <= 0:  widthToColumnWidth = 0
        Case Is <= 12: widthToColumnWidth = pts / 12
        Case Else:     widthToColumnWidth = 1 + (pts - 12) / (75 / 11) '
    End Select
End Function

Public Sub HSVtoRGB(h As Byte, s As Byte, v As Byte, r As Byte, g As Byte, b As Byte)
    Dim minV As Byte, maxV As Byte, Chroma As Byte, tempH As Double

    If v = 0 Then
        r = 0: g = 0: b = 0
    Else
        If s = 0 Then
            r = v: g = v: b = v:
        Else
            maxV = v: Chroma = s / 255 * maxV: minV = maxV - Chroma
            Select Case h
                Case Is >= 170: tempH = (h - 170) / 43: g = 0
                                If tempH < 1 Then
                                    b = maxV: r = maxV * tempH
                                Else: r = maxV: b = maxV * (2 - tempH): End If
                Case Is >= 85:  tempH = (h - 85) / 43: r = 0
                                If tempH < 1 Then
                                    g = maxV: b = maxV * tempH
                                Else: b = maxV: g = maxV * (2 - tempH): End If
                Case Else:      tempH = h / 43: b = 0
                                If tempH < 1 Then
                                    r = maxV: g = maxV * tempH
                                Else: g = maxV: r = maxV * (2 - tempH): End If
            End Select
            r = r / maxV * (maxV - minV) + minV
            g = g / maxV * (maxV - minV) + minV
            b = b / maxV * (maxV - minV) + minV
        End If
    End If
End Sub

How to run this in Excel:  Paste the code above and paste into a regular module. (Select code, Ctrl+C to copy, then in Excel, hold Alt and hit F11+I+M and then Ctrl+V to paste, and F5 to run.) 

More Information:


*

*Wikipedia: HSL and HSV 

*Wikipedia: Color Wheel - HSL

*colormatters.com : Basic Color Theory

*Wikipedia : Midpoint circle algorithm

A: If you want a color wheel like the example you provided (and like most color wheels you'll find in a craft store paint section), where red is opposite green and blue is opposite yellow-orange, purple opposite yellow, etc., you can simply do the following math to modify hue of HSL or HSV to get the legacy hue...
double ToLegacyHue(double modernHue) {
   modernHue = ((modernHue % 360) + 360) % 360; // normalize 360 > modernHue >= 0
   double ret = 0;
   if(modernHue < 60) {
      ret = modernHue * 2;
   } else if(modernHue < 120) {
      ret = modernHue + 60;
   } else {
      ret = (modernHue - 120) * 0.75 + 180;
   }
   return ret;
}

double FromLegacyHue(double legacyHue) {
   legacyHue = ((legacyHue % 360) + 360) % 360; // normalize 360 > legacyHue >= 0
   double ret = 0;
   if(legacyHue < 120) {
      ret = legacyHue / 2;
   } else if(legacyHue < 180) {
      ret = legacyHue - 60;
   } else {
      ret = (legacyHue - 180) / 0.75 + 120;
   }
   return ret;
}


A: Have a look at http://www.easyrgb.com it has the algorithms behind many color conversions. Here's the RGB -> HSV one.
var_R = ( R / 255 )                     //RGB from 0 to 255
var_G = ( G / 255 )
var_B = ( B / 255 )

var_Min = min( var_R, var_G, var_B )    //Min. value of RGB
var_Max = max( var_R, var_G, var_B )    //Max. value of RGB
del_Max = var_Max - var_Min             //Delta RGB value 

V = var_Max

if ( del_Max == 0 )                     //This is a gray, no chroma...
{
   H = 0                                //HSV results from 0 to 1
   S = 0
}
else                                    //Chromatic data...
{
   S = del_Max / var_Max

   del_R = ( ( ( var_Max - var_R ) / 6 ) + ( del_Max / 2 ) ) / del_Max
   del_G = ( ( ( var_Max - var_G ) / 6 ) + ( del_Max / 2 ) ) / del_Max
   del_B = ( ( ( var_Max - var_B ) / 6 ) + ( del_Max / 2 ) ) / del_Max

   if      ( var_R == var_Max ) H = del_B - del_G
   else if ( var_G == var_Max ) H = ( 1 / 3 ) + del_R - del_B
   else if ( var_B == var_Max ) H = ( 2 / 3 ) + del_G - del_R

   if ( H < 0 ) H += 1
   if ( H > 1 ) H -= 1
}


A: steps for drawing color wheel


*

*take a point with cartesian coordinate : (x,y)

*convert cartesian to polar : (x,y) -> (r,theta)

*convert coordinate to color = polar to hsv : (r,theta) -> (h,s,v) 


where: 


*

*theta = hue

*radius r  = saturation

*value is const 


Here is a tool and nice examples 
