how to remove color of light source?
Do you want to remove the Yellow backgorund or make the Yellow light white?

If you want to color correct the image so that Yellow appears white then simply measure the R,G,B values of 'yellow', calculate the ratio between them. Then scale all the pixels in your image by that ratio.

Typically you would assume that green = brightness and just calculate red/green and blue/green then scale the red/blue pixels and just scale green to leave the overall brightness unchanged.

See http://en.wikipedia.org/wiki/Color_balance