Here is an example of a tileset I've made:

The colors you've chosen aren't bad, but here are some tips:
-If you use shading, which you should, don't just use a darker color, change the hue a bit too, so that if you want a darker red, you pick a dark purple-red. It'll look great.
-The middle row of your tileset (the dirt without grass on it) has no texture. It'll look pretty bland in games/etc. You should give it a texture like dirt/sand or whatever, but don't make it too spectacular. The focus shouldn't be on the tiles.
-If you put grass on that dirt texture (should loop), it should make a shadow (realistic). The grass should be not that linear too. Looks too digital.
I think it's great that you're making tiles, since there are way too less of them on the web and we always need them.
Damn, I need to make a tutorial