Its all about accessablility.You have to specify a background color with your color, and visa-versa.If you dont want a color or background colour, use :inherit;this will validate against W3C.Also check if its an error or a warning, if its a warning this is still valid code, but it p**ses me of that you cant have one with out the other.
I belive that 'transparent' is only for fx, it wont work in ie, suprise spuprise,