sounds like ur completely new to gimp...
okay for the first to get the layer to multiply i'm assuming u will need the make the duplicate of ur first layer... then just press Shift+L and the layers box will come up.. there u can duplicate the layer then change it to multiply by goin to the box with should say "normal". just press the arrow pointing down next to it and u can change it to multiply.
and to change colours u just need to double click on the black or white colour box. then u can choose ur colour from there.
hope that helps.. sorry if my explanations aren't clear.