Cannot draw over Color on the same Layer

[Gimpquester]

I must have changed the default setting somehow and now am clueless as to how to get it back. 

Drawing on a new layer with a color using paint brush works just fine. When I try to add a new color now however I can only draw on the empty space of the layer, and no longer over the color. It's as if the old color had become a seperate layer that's laid over my layer. See the two images attached.

Thanks for your help.