Try cleaning the screen with a slightly damp cloth to see if this solves the problem. If not, then you may need to contact Nintendo and let them know about the problem.
then take of the sheild cover on the back of the DS
to reveal wat should be the RAM type graphics processor
a small chip
NOW TO BE SAFE
make sure it's off then check the wiring
if it's all ok then you may need a replacement chip
EITHER THAT OR THE COLOURS (OR PIXELS) BEING PROCCESED ARE COMING OUT WITH ONLY BLUE AND GREEN AND NO RED E.T.C...
SO THE "COLOURCHIP" IF YOU LIKE, IS ONLY PROCESSING 2/3 PRIMARY COLOURS GIVING YOU THE YELLOW COLOUR OVERLAYING THE SCREEN.
take a look and see wat you think could be wrong som way