To be able to increase the quality of my paintings, especially rendering, values and color, I decided to do more color studies.
In doing so, you "copy" a photo or something from life without using the color picker in photoshop. This is essential, because you are forced to judge values and colors with your own eye, hereby training it to see.
As I mentioned before, it is really important to NOT blindly copy the image on autopilot, but observe the image and try to memorize why something looks the way it does under certain lighting situations. Here are some examples:
Best results for memorizing are achieved, if you use reference for something you want in a final image, study (paint) the reference material and apply to you own images, e.g. you want to paint leather armor on a sunny day, find reference (no need for it to look exactly the way you want, it is all about studying how something looks), paint it without color picker and apply what you learnt to your own supercool armor.