Back gummetal spray with something before red goes on?

[sosidge]

I'm trying out a new colour scheme...

Tamiya PS-23 Gun Metal has already gone on the back of the car. The rest of the job is a red stripe around the edge of the gunmetal, then a white nose. All Tamiya sprays.

Now, should I back the Gun Metal with something (perhaps the white) before spraying the red? Or will the red go over this darker metallic without affecting the shade too much?