How can I change the color on my name/picture in lobby and games?
I don't think you can change the actual colors in the settings. You can turn them off or select a preferred color in the lobby, but without going into the files and changing it manually (which i don't recommend) it isn't possible.
EDIT: Here are the patch notes from when the teammate colors were introduced. There is no mention of an option to change it in this or future patch notes.
You can create a lobby, right click on your picture* and change color. It'll stay in that color in that lobby & the games you enter with that lobby. If you aren't 5/10 (5 for Competitive, 10 for Casual) in a lobby and somebody else selected same color too (in another lobby), your or his/her color will be random.
//add this to your autoexec.cfg... create file if it does not exist in steam_folder/steamapps/common/Counter-Strike Global Offensive/csgo/cfg)
cl_color 0 // for yellow, cl_color 1 = purple, cl_color 2 = green, cl_color 3 = blue, cl_color 4 = orange
host_writeconfig // if that doesn't set it, run exec autoexec.cfg from console and restart