Probably not, unless using an external probe (like those used in color calibration) or using advanced color computation with the current ICC color profile, color profile of the monitor you use, etc.

If you meant to read the logical color as painted in the sketch, you can use get().

Maybe you can help me if i tell you what i was thinking about. I just want to use the led strip as a background lighting, which adopt the actual color of my screen.

yes, as said get

get(x,y);

http://www.processing.org/reference/get_.html