Most likely the color coding of the memory slots is for dual channel support. If you need dual channel support (increased effective memory speed but reduced memory) then you put two identical memory modules in the same color coded slot. In this case amount of memory does not get doubled, rather, speed gets doubled (theoretically because double bandwidth). If you want to increase your memory, then you must put one module in one color slot (say Blue) and other one in the different slot(say black). This forces single channel mode.
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Increasing the total RAM to 6GB will not be of help in 32-bit OS, unless you are using 64-bit OS