Great pic! Tarantulas would have first developed scopula for climbing, and colors probably came later. With their fang arrangement, which requires stabbing downwards, colors are a nice evolutionary 'add-on' when standing up in a threat display trying to make a warning as dramatic as possible. Red around the mouths parts adds to it. One of the most impressive displays in the tarantula family is the black and yellow ventral leg bands on Poecs, using the warning colors/stripes of bees. Black/yellow bands, alternating colors of blue and green on the scopula when they move, red mouthparts, and fangs extended. Wow. If that doesn't stop your advance, don't expect any mercy. You deserve what you get.The iridescence is a product of the very dense setules on the scopulae. This actually splits the light into it's colors like a prism. It gives the spider the ability to adhere to smooth surfaces and grip objects it touches with virtually no effort, like tongs lol. Here's a picture I just took of my S. calceatum's molt at 60x and it's pretty clear what's going on.
The angle at which the light source is directed at the scopulae determines what colors you see. I had the light hitting from around 90°, and it looked like this. When I rotated the light source to closer to 270-300°, it changes to green/yellow.
There are so many fascinating things about tarantulas, it'll take decades to figure out a lot of them. How did they ever come up with digesting prey before swallowing it? Or using a secondary device (pedipalps) for sperm delivery? How did that come about? They're covered in sensory organs, the uses for silk are incredible. What advantages silk gives them! Stronger than steel, and yet super flexible.This is a really interesting and informative thread. Thank you all for taking the time to teach this to those that are still learning!