Knew this was going to happen.
Top pic is the second colour applied - "Vallejo Luftwaffe Grey". And, as i thought, its a horrible clash: the one shade dark against the one shade light. Know that once i start on it, it'll pull together, but that voice on my shoulder kept up the "you've made a msitake, you should've stayed with one colour".
Felt better taking the second pic. Here i've mixed the two greys and splodged it on, varying the amount of the lighter colour each time. Still a way to go, but its getting there and i'm feeling better about it. Will feel even better when i've a put a dirty wash over it to pull everything together and break up the light bits.