Thanks mate, that was it, Ctrl+L.
So I have opened a battery png for a tester. I wish to change he colour from green to blue. What is the easiest way?
I simply wish to change green to a selected colour blue.

I understand as above noted, tuts on the net. I have seen a few and pretty in depth, I didn't expect to read or watch hours of tuts to see if I can find how to change a colour. Hence why i posted the thread, just thought someone that is very good with the software would give some quick simple advice. Click this, do that, colour changed, maybe not that simple with GIMP?