Motivated by the movie, I wanted to make it quickly, and since the white parts are mostly straight lines, I decided to color them in with a marker, so I used the highly acclaimed Gundam Marker EX New White. 
The blue and navy undercoats don't show through, and the color comes out amazingly well in almost one stroke!! 
The paint film is weak when painted with a marker, but it's actually easier because you can fix any excess paint with a bamboo skewer or something without damaging the plastic underneath.

Gundam Marker New White is amazing | スポーン's Gunpla works