very nice concept
i really liked this idea of the colours, it kinda reminded me of antbuster td, with less variables, but overall a very original idea. As some people before me have said, it gets very laggy on later levels when there are lots of towers firing off at the same time. but apart from that, the gameplay id very nice, and i especially like how you can buy back lives, but they cost more and more as the levels progress.
Tips, in all seriousness, all you really need is the flame and ice towers for the first 10 levels or so, then just move onto lightning, wind and ice towers. also, a level 3 gold tower will pay itself off in about 3 rounds, so it is very usefull in the long run.
While on the topic of towers, it would be nice to have some kind of Key that told you which towers can attack which kinds of units.
i like the graphics, and, although the gameplay is somewhat repetitive, this is a flash tower defense game, so there is nothing that could have done to avoid that.
overall 7/10, fix the lag, and maybe make a colour upgrading system for each tower instead of all towers together, and this game will be 10/10