You’ve got two towels made from the same material, except one is red and the other is blue. What, at the smallest possible level, is happening on the towels that makes us see them as different colors?
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Are you asking what dyes are? Think of food coloring but for fabrics. It’s just a pigment added to the fabric that stains the fabric. Cotton for example is white naturally, anything that is cotton but not white (even white might be bleached) is stained with a dye and that is why it’s a certain color you see.
At the smallest possible level?
Dye molecules in the towel absorb photons from the sun, gaining energy and exciting their electrons. When the electrons stop being excited, they will release another photon to shed the energy Depending on the dye molecule, that photon will be a specific wavelength and colour.
You can safely ignore the towels and go to the dye molecules. Dye molecules are chosen because their chemical structure leaves a gap in electron levels that corresponds to energy levels in the visible light range. https://en.wikipedia.org/wiki/Dye
Dye molecules have color because they absorb certain other colors of light. So more or less when a red dye is hit by white light (which is made up of the other colors of the rainbow), only red comes back.
They absorb different colors because of their molecular structure. Different colors of light have different amounts of energy in each photon. The electrons in a molecule have a set of levels they can be. If a photon hits a molecule that matches its energy, it can be absorbed. It’s like if you can only jump to another stair step if you hear the right musical note, because the pitch you hear corresponds to how high you can jump, and you can’t jump over and just fall back down.
Pretty complicated explanations for a five year old.
Light is white because it contains all of the colors. When it falls upon an object, it absorbs all of the colors except the color it is, it reflects that color and our eyes see it.
Now, go outside and play.
The two towels have been colored with different dyes, one red, one blue. Think of the different color dye molecules as two different frequency tuning forks. (This explanation involves so much handwaving that it’s completely wrong, but there’s no real alternative that doesn’t involve explaining the photon picture of EM waves, electrons’ energy levels in a molecule, why those are discrete instead of continuous, why then everything still works even if the light isn’t at exactly 450.00603nm, etc.) Incoming light waves make them vibrate, but because of complicated quantum mechanics reasons they do so at certain frequencies much more readily than others, and that vibration emitting light waves is what you see, just like tuning forks can be excited by sound and also emit sound.
As for how to determine what the frequencies are, that’s also down to complicated quantum mechanics. Chemical dyes tend to have long conjugated systems (carbon chains with alternating single/double bonds), though that’s not the whole story. Historically, many dyes were invented way earlier than QM, so it was a process of trial and error rather than calculation.
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