Answer to: Color problem with nesMost likely a problem with either the PPU chip in your nintendo or on the video output side. I'd open it up and check to see if it's clean in there. 75% of the consoles I've fixed have some pop drink spilled onto it. I'd clean that area, if not I'd reflow (resolder) the pins on the ppu. If that doesn't work it could also mean some bad caps.
Answer to: Yellow Tinted Bottom Screen on DS LiteIt's normal, the plastics for the screen tend to yellow with age. You could replace the protective plastic covering the screen.