Another possible option (apologies if you've already thought of this) is to paint the whole thing black first (or just paint a wide black line but that risks uneven tone in the other colours later). Then mask your pinstripe with narrow masking tape. Then paint the whole thing white. Then leave the pinstripe masking on and mask the white side off - which will be easier because you have the pinstripe width to overlap the masking on - then paint the blue.
Harder to describe than do
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