"the Vallejo stuff", Are you brush painting or airbrushing?
Vallejo makes several lines of paint. Model Color, Game Color, and Panzer Aces (among others) are made for hand brushing. Model Air and Game Air are pre-thinned for ease of airbrush use. Model Air/Game Air will be thinner and harder to get coverage when using a brush. Multiple coats will be required
The above suggestion to add some white/light gray is good, but consider the body of color paint when you are making your paint selections