To change the color of the representation you need to use the tolls in the display tab.

Changing the colour of a representation works in much the same way as displaying the representation itself. The selection rationale is the same followed by clicking on a colour in the palette. It is also possible to colour different representations of the same selection independently (e.g. when displaying a loop (selection series of residues), represented as ribbon and xsticks; colour the ribbons of that selection in cyan and the xsticks in red).