Long story short, I tried to make myself a nifty TARDIS banner in Minecraft PE... After I covered ink sac patterns with lapis lazuli patterns, the result was still darker than it should've been... Notice that the MCPE in-game TARDIS has darker outlines than the other version... This might work if you cover ink sac patterns with other dyes, too, although I haven't tested that out.
MCPE-23980
Black lines on banners are not the right shade after lapis lazuli is applied to them
Resolved
Fixed
2
(Unassigned)
Unconfirmed
Phone - Android - LG Nexus 5
Android 7.1.1
banners
1.2.0.2
1.2.0.11
Retrieved
Another thing I realized was that using gray dye instead of an ink sac doesn't solve the problem: the lines stay the same shade!