It's HIGHLY adviseable to always use indexed palettes or at least reduce it as much as possible.
An image with only one color would essentially only require 1/32 of the space the 32 bit image uses (that's not entirely true but it's the rough dimension).
And as dhwz said, the number of colors actually used doesn't make a difference.
A 32 bit pixel always uses 32bits for color information (ususally it's "RGBA 8888").
Especially on the dm520 that makes a huge difference.