yes, it is, but it can only be used in very specific conditions (you cannot have the colors where you want all over the place), it's a pure hack.

You could either set any pixel color to any of 16 palette entries OR hold red & green from left side pixel, modify blue, hold red & blue, modify green, etc.

You could also change those 16 colors for every scanline (horizontal line).

Overall there was a lot of control, but software such as Deluxe Paint did not support full possibilities of HAM mode.

Oh and, btw, EHB (extra halfbrite) mode has 64 colors. 32 colors palette + second 32 colors with brightness halved. http://en.wikipedia.org/wiki/Amiga_Halfbrite_mode