There’s a lot of science involved in art. A lot. But most of it happens behind the scenes and isn’t really anything that you need to know about.
And yet, here you are.
So let’s start from the basics: what is colour?
‘Colour’, is the way our brains interpret light waves. Our eyes have these things called rods and cones, which interpret light waves. It’s the rods we’re interested in, they’re the ones that register colour waves, and we only have three types: Red, Green, and Blue. Our brains receive incoming light waves and deduce through how many of each colour rod is ‘activated’ what the light wave is, which is then interpreted as a colour. Most people interpret light waves the same way, some people have colour-blindness where their eyes interpret light waves differently. A common form of colour blindness is red-green colour blindness, where the red and green colour rods in the eyes register light waves inconsistently with ‘the norm’, and so a more limited visual colour palette is interpreted. Some people don’t see colour at all, but see everything in black and white.
In it’s simplest terms, we see light waves from the sun (I.E. light waves in the ‘visible spectrum’ of 380 to 700 nanometres) reflecting off the stuff around us. Depending on the chemical make up of the object, light waves are absorbed or reflected, and our brains interpret the reflected light waves as colours. A leaf, which absorbs sun light for photosynthesis, reflects the light waves around 495-570 nanometres, which our brain interprets as green. An orange, reflects light waves of 590-620 nanometres, which we see as orange.
Different animals interpret the same light waves differently - like dogs, who see everything in greys, blues, yellows and browns  - and others can interpret a different range of light waves - like bees, who can see fewer reds, but can register colours well into the ultraviolet range of the spectrum . Mantis shrimp have up to sixteen (SIXTEEN!) colour rods compared to our three, and frankly I think that’s a bit selfish (shellfish haha) of them, though that doesn’t necessarily mean they see more colours, which is weird, but, science.
There are two types of colour mixing - additive and subtractive.
Subtractive colour mixing is for projected light - the sun, a computer screen, the light bulb in your bedside lamp.
With subtractive colour mixing, all the wavelengths combined give a pure white light, and none of the wavelengths gives, well, no light at all.
Additive colour mixing is what we do as artists - it’s combining colours or pigments to reflect the light waves that we desire. In additive mixing, the absence of colour pigment (I.E., a paint that absorbs no light waves and reflects them all) gives white, and the more colour you apply, the further away from white you travel. Combining all the colours together so that all the light waves are absorbed and nothing is reflected gives you black*.
So. Why is this relevant to you, my artistically inclined companion?
Well, it’s to do with your painting.
I’m sure you’ve seen on the back of boxes for light bulbs that they come with a ‘colour temperature’ gauge.
Warmer light, like candle or firelight, sunsets and sunrises, or the old sodium-vapour street lamps can neutralise cooler colours and makes them seem darker and less clear. This is because these light sources lack the light waves needed to reflect the colour of the stuff. So if you have a bright green plant under a very orange light, it’s going appear a rather browner green. Similarly, if you hold an apple under a very cool, blue light, it won’t be the bright shiny red that it would be under a neutral light, it’ll be, surprise surprise, a brownish red.
This is why it’s so important when you’re painting to make sure that you’re using the correct kind of lighting, such as a daylight or neutral bulb which emits the full spectrum of light waves, which, if you're looking on the back of the box, would be in the 5000 - 6000 Kelvin area of the colour temperature gauge.
To summarise then, colour is the interpretation of light waves by the human brain, and therefor entirely subjective based on the lighting and environment you’re in. The same object of the same chemical makeup can be entirely different colours, depending on when, how, and under what light look at it.
*Side note, black is actually a really interesting colour, and there’s even a bunch of controversy and scandal around it, so we’ll talk about it at some other time.
References and useful reading material