I'll assume this is a default grey object and you don't want to apply a texture to it just a change of colour.In the matterials editor you'll see some ovals next to the words Diffuse/Ambient/Secular etc.On diffuse (you may need to repeate this on ambient and the others)left click the oval (grey patch) and a colour slider will appear.Keeping the mouse button down move the cursor arround in the colour area until you get the colour you want in the oval that you clicked.Let go of the button and the colour has changed.