Okay, as a painter, I have painted, inside, in the daytime with only light from outside coming in and in the evening, with an overhead light fixture.
I have also done plein air painting, which is a different kind of light, because it is direct sunlight, which may or may not hit the paper and/or the landscape that I'm painting. In plein air, one can and usually does, get up close to the tree, flower, grass, whatever, to check color, textures, minute details ( botanical work is often detail happy! ), so yes, different kinds of light, different times of day/night, but I don't see the difference.
If I mix every color ( though painters usually do NOT do that to get black ) yes, it will come out sort of black, IF I omit Titanium, Buff, and/or Chinese White.
Under NO circumstances, that I work under, can I mix all colors and get white.
And...most watercolorists DO NOT use white paint. If we do, it is NOT the white watercolor; it's Dr. Ph Martin's BLEED PROOF paint. The other way we get white, is to use masking fluid, so that when that substance is removed, the sterile, white paper is exposed. But....since most white things or highlights are NOT pure white, we use a different method, entirely, which is using the MOST watered down paint to differentiate certain portions of the subject, leaving white paper areas exposed.
LOL...probably too much info above and perhaps NOT the answer you were looking for. But it's the best I can do.
Thank you for your thoughtful reply.
White pigment paint absorbs no visible wavelengths which is why it not only appears "white", but also why surfaces painted white don't get hot in the sun, whereas surfaces painted black absorb all the visible wavelengths which is why they get hot in the sun.
That is the very definition of "reflective color".
There are, as you no doubt know, many proposed mechanisms to translate between pigment colors and RGB monitor signals (e.g. Pantone). Artists who work on computers for websites have faced this problem for a few decades, and it's still not a solved problem -- there are only approximations.
Take an apple. Paint a picture of it. Take a photo of the painting. Display it on a monitor. Print it out on a high-quality color laser printer. They'll all be subtly different.
For millennia, humans experienced almost all colors as reflected light -- a sunlit garden, an apple tree, the face of a lover -- and our brains got very good at discerning color that way. The only additive light sources were the Sun (and you better not look directly at it), fire, lightning, fireflies, etc. which are all very transient. Even electric light bulbs -- who stares at a lightbulb expecting to see something? We only use it to illuminate reflective objects.
The introduction of TV displays and computer monitors changed that, and transmitted light sources are now common everywhere. Artists are still tearing their hair out trying to get a computer image to look exactly like their pigment-based creation.
As one trained in physics, but having many family and friends who are artists, I have watched this evolution for the past decades with great interest.