function extendedSinebowColor(i, a) {
        return i <= BOUNDARY ?
            sinebowColor(i / BOUNDARY, a) :
            fadeToWhite((i - BOUNDARY) / (1 - BOUNDARY), a);
    }
Interpolates a sinebow color where 0 <= i <= j, then fades to white where j < i <= 1.

@param i number in the range [0, 1]
@param a alpha value in range [0, 255]
@returns {Array} [r, g, b, a]