Not really a rainbow.

RAINBOW.go

func RAINBOW() []color.Color {
	const (
		paletteSize = 256
	)
	var palette [paletteSize]color.Color
	// 1 0 0 : RED      1  +
	// 1 1 0 : YELLOW   0  -
	// 0 1 0 : GREEN    2  +
	// 0 1 1 : TEAL     1  -
	// 0 0 1 : BLUE     0  +
	// 1 0 1 : PURPLE   2  -
	// 1 0 0 : RED
	stepSize := paletteSize / 6
	stepExtra := paletteSize - (stepSize * 6)
	colorStep := 255 / stepSize
	colorExtra := 255 - (colorStep * stepSize)
	place := []int{1, 0, 2, 1, 0, 2}
	col := [3]uint8{0xFF, 0x00, 0x00}
	var j int
	palette[j] = color.Black
	j++
	var p int
	for p = 0; p < len(place); p++ {
		for i := 0; i < stepSize; i++ {
			palette[j] = color.RGBA{col[0], col[1], col[2], 0xFF}
			// increment color
			col[place[p]] += uint8(colorStep)
			j++
		}
		if colorExtra != 0 {
			col[place[p]] += uint8(colorExtra)
		}
		colorStep = -1 * colorStep
		colorExtra = -1 * colorExtra
	}
	p = 0
	for i := 0; i < stepExtra-1; i++ {
		palette[j] = color.RGBA{col[0], col[1], col[2], 0xFF}
		j++
	}

	return palette[:]
}