White means you're not really doing any notable enhancement, Green means YAY you're enhanced. Yellow means you're nearing the capacity of that type of enhancement and further enhancing may be wasteful; and red means you've over enhanced and you're probably using a fraction of that last enhancement you HAD to put in, usually resulting in a wasted enhancement and/or a wasted power slot.
Apologies for the "genrico" explanation... as far as I know that's what the colors stand for.