coloring, size change, & make geometric figures in GIMP layer

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http://www.linuxquestions.org – Have been reading about GIMP now till my eyes have fallen out. So far I haven't discovered how to change color of my brush strokes on a single layer. Can someone help? Also I need to make circles and drag them to a specific location on a layer. I also need to make various sizes of circles some about 1/2" others 1/8" diameter (yea that depends on the size of the screen etc. ...) but U get the idea. Could someone help here? BTW is there a way to make the pencil width or brush width smaller/larger? (HowTos)