Grey hair help!

I have brown hair with blonde hi-lights, the problem is when the hi-lights grow out a bit my pre-mature grey starts to sneak through and looks awful, what can I do? I'm not sure if I should use a temporary color if I should be putting more chemicals on my hair, or should I find one of those hair mascara things? It just seems like it's too soon to have to go for hi-lights again, plus they are pretty pricey. Any advice would be great! TIA!