In my art classes this past week, the focus was on colour and colour mixing. There’s always some great discussion and the endless possibilities of colour amaze. I was reminded of a question my friend John had asked not long ago. He’d just finished reading the book “Yellow & Blue don’t make Green” by Michael Wilcox, and wrote me:
“Here is a puzzle for you: Yellow paint is yellow when illuminated by white light. Why? Because it absorbs all other constituent colours in the white light, and reflects only the yellow frequencies back to your eye. Likewise, blue is blue because it absorbs all colours but blue. Black is black because it absorbs all. So if you mix blue and yellow paint, you should get black and not green. It should be black because the reflected yellows from the yellow pigment should be all absorbed by the blue pigment, and the reflected blues in the blue pigment should all be absorbed by the yellow. So why does it make green? The book explains this, but can you?”
A fine real-world versus theory question.
A yellow surface looks yellow because the reflected spectrum is centered on the yellow…the spectrum being reflected is modified from white because less (not all) in the red and blue range are reflected. In the real world a relatively broad spectrum of wavelengths is always reflected: if you could concoct a “pure yellow” (single-wavelength) reflector (not an ounce of red nor blue) and combine this with a “pure blue” reflector (not one whit of yellow nor red), this would, indeed, in theory, look black!
I got the Wilcox book from the library and it explains this well, noting that colours do not “mix” to form a third. It’s just that the resultant reflected spectrum is centered on the wavelength of “mixed” colour. That is, the yellow and the blue still exist in the green, but they don’t somehow morph into green.
Now if this all sounds like mud, let’s try making purple. Why is it that almost any yellow and blue will “make” a usable green, but usable purples are far more elusive. Use the “wrong” red (an orangy-red) and you’ll be stuck with a muddy mess.
To answer this, I may need to wade into the dreaded debate over red-yellow-blue primaries versus the cyan-magenta-yellow (CMYK) model.
Sparks could fly…more to follow!