What sound card are you using and what main color are you planning on having? Red and Green can be combined to a decent yellow, the thread here is a bit old, but it describes colors with which you might be able to make a brighter yellow.
If you are using a card that adds the flash to the main color (pico crumble, nano biscotte), the only way that could really work is to add red to green, or green to red.
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