For a WIP, I'd say it looks good. What are you using purple things for?
I've been using GIMP a lot to alter existing stuff. The only things I've made which required a lot of "hand-made" graphics are icons, and a few characters.
To echo what the poster above said, yes it's not always easy to get it to fit in with other default resources. I sometimes find it looks better if you make the graphic you want in your game, and then use a tool (like GIMP) to drop down the contrast and brightness a little bit. I'd avoid using the darker shades of colors as the default graphics (to me) seem to avoid the extremes, and never have too high of a contrast.