color depends on the strain. Get a can of spray paint if you want to change their colors. Increased yield...Light, light, and more light. Do everything possible not to stress the plant out at all. Keep temperatures/humidity in optimal growing/flowering conditions. More light. Co2 rich, fresh air. Potency is strain dependent and also depends on when you harvest. I've heard using UV-B lights can increase resin amounts. That's ultraviolet radiation...It can be health hazardous to your eyes and skin. UV-A is the safer black light, but it supposedly doesn't do the job like the ultraviolet rays the sun puts out. I have yet to test this out so don't take this for certain. The increase in resin, if any, is probably barely even notable anyway.