Doc,

Have you ever tried cobalt blue? I ask because the phthalos in general are very strong pigments (i.e. hard to clean, and hard to cover) and are transparant compared to the cobalt (semi-transparent). That explains the two coats, but it might look 'brighter' with the base showing through after two coats (your eyes will not see the white). However it may be that intensity that works best and/or might original for that brand sign.