Stick with me on this... it may seem boring but in order to understand color relationships and what "matches" you need at least a basic understanding of the color wheel. It'll be painless and have lots of images!
The traditional 12 Spoke Color Wheel (not the color wheel for web based projects) starts with 3 primary colors. These colors cannot be made by mixing any other colors - thus, the primary status.
Yellow, blue and red are the three primary colors. The positions of these primary colors are illustrated in the image, left.
Our "color wheel" is depicted with two triangles to better explain color relationships.
When you mix equal parts of 2 Primary Colors together, you get 3 Secondary Colors. These new colors are inserted on the wheel between the two Primary Colors that were mixed. So, your color wheel now looks like this:
Mixing yellow and blue, you get Green.
Mixing red and yellow, you get Orange.
Mixing blue and red, you get Violet.
The next step is mixing equal parts of the two colors that appear beside each other on the wheel to get the 6 Tertiary Colors. These are then inserted between the two colors that were mixed together to create each Tertiary color, with the primary color used listed first in the new color names.
Their formulas are:
Yellow + Green = Yellow-Green
Yellow + Orange = Yellow-Orange
Blue + Green = Blue-Green
Blue + Violet = Blue-Violet
Red + Violet = Red-Violet
Red + Orange = Red-Orange
images source: Mandee Sears