paint backing query

[dimmockg]

go easy, first time shell painter here

doing a scheme on a shell, where i have black on the top half of the body - using a pre applied mask, which has a border. Will paint the black etc, pull the border of the mask, paint silver, then paint the middle

thats the easy bit

on the bottom half off the body, i'd like white as the main colour (which will be comlpetely masked prior to painting the black/top half)

what i'd like on the bottom is a pre-applied mask/decal with a border, so do i do the following... (after finshing the top half of the shell)

1) remove the masking (leaving the decal/mask)
2) paint white
3) remove the border of the mask/decal and paint black - this is where my query lies
4) remove middle and paint light blue/purple etc what ever colour i decide

basically the bottom half of the shell is inverted from the top, but i've read that painting the black (for the mask/decal border) on top of the white isn't a good idea. Is there a way round this? i've thought of masking around the edge of the applied mask/decal, but it would be too complex

i did think about back the white with silver, then white, then applying the black but not sure if this can be done either?

the required black will only be approx 2mm thick and be the border of a tribal type design covering an area of approx 1 inch by 3 inches

cheers