My wife asked me a year ago why I only own blue shirts.
It is actually for landscape painting. If you paint with the canvas in the shade you are probably standing in full sunlight (I don’t use an umbrella), in which case the color of the shirt will reflect on your painting and affect the hues. A few years ago I had a favorite orange T-shirt my brother gave me, and a couple of times I went painting with it on and had to go back to the house to change as the reflected orange was so distracting. I’ve also tried with black, but if I’m up against a hedge or in an area with very little reflected light I find the canvas can be too dark. White can also be annoying as you see your shirt reflected in the painting.
At the end of the day, a sky-blue shirt is the best for plein air landscape painting. I like mine with long sleeves in case of excess sun or mosquitoes. A shirt-pocket is useful for glasses or my superior Korean ipod.