Puzzled By SetColorTexture

[Firesong]

This question is not crucial but I am puzzled and would be grateful for a pointer in the right direction.

MapFrame.texture:SetColorTexture(0, 0, 0) gives me black.

MapFrame.texture:SetColorTexture(255, 255, 255) gives me white.

MapFrame.texture:SetColorTexture(255, 0, 0) gives me red.

MapFrame.texture:SetColorTexture(0, 255, 0) gives me green.

But MapFrame.texture:SetColorTexture(12, 34, 56) gives me white as do a whole lot of other combinations.

I've confirmed the unexpected whites by screenshot, paste into Paint and using the color picker.

How can this be? Why are combnations like 12, 34, 56 giving me white?

Thanks in advance.