The Visible Color Spectrum
White light is a mix of all primary colors (ROYGBV). Remember prisms from elementary school? See below for how Red, Orange, Yellow, Green, Blue, and Violet (ROYGBV) can be separated from white light! Important point: Violet does not mean purple. It is the "roses are red, violets are blue" color.
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Wild Violet Flower
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Picture source: http://blogs.nature.com/ofschemesandmemes/files/2015/02/Week-2-Blog-Dispersive-Prism.jpg
ROYGBV is just the visible light wavelengths!
Your Body's Light Penetration
You can see red light penetrate through your finger if you put it in front of a flashlight. You see red on the other side because most of the other colors were absorbed in your finger. Try this at home! Put your phone flashlight, or any white flashlight, on one side of your finger. Then look at what color makes it to the other side!
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www.researchgate.net/publication/316712743/figure/fig13/AS:668385360744448@1536366896067/a-Tissue-penetration-depth-of-light-with-different-wavelengths-NIR-light-penetrates.png
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https://images.theconversation.com/files/201297/original/file-20180109-83559-19kt3yi.png?ixlib=rb-1.1.0&q=45&auto=format&w=754&fit=clip
Red light can be used to see inside your body.
Well, somethings not right with this finger. Got an x-ray and yep, something is there.
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All images below are sourced from: http://www.eatonhand.com/img/img00087.htm
Let's shine red light on it. Yep, it's darker where something is sitting. That means there is not good blood flow there, so there is more deoxygenated blood in that spot.
Click here to see what was taken out after surgery. Warning: somewhat graphic image.