Well those colors are horrible. Try make it black, then make it shine with some white brush work.
Well tropical, should be 3 colors. To make it bit more round, try adding tints and tones of the colors, for example, darken the colors a bit around the edges, and maybe a shine spot or two around the center.
Its a good sketch it just depends on what kind of spider it is..then once you choose what kind it is then reserch it on the internet and find out what colour it really is (IRL) and there ya go.