I agree with CR, and those colour splotchy things Revell use are about as useful as an ash tray on a motorbike - even on the printed page! If you're going to mix your own Keith, get a mid grey, a bit like the colour of grey car primer, and add a touch of green, so that it ends up grey-green. The colour shown on the Revell splotch, as seen, is reasonably close for a form of RLM 02, (although the actual paint might not be) so you can use that as a guide.
Even though RLM 02 was the same colour used as an upper surface camouflage, if you look at even black and white pics of the period, say around the time of the BoB, the shade in the inside the cockpit of the '109E often looks lighter (slightly) than the outside, and more of a grey tone. It really is one of those things where no one can say for certain you are wrong - or right! Also, on a model, where the physical area of the cockpit is small, it's normally better to make the colour a bit lighter, to counteract the lighting effect. If it looks right, it is right nearly always works!