The eink controller docs say the DU (direct update) and A2 (animation) modes are intended for things that need to change fast, like a touchpen cursor. Those modes update to saturated black or white only (no gray pixels in the update region). In fact, for the Broadsheet eink controller (K2/DX/DXG/K3) displaying any gray pixel in DU mode converts it to white. Pure black and white complete the update MUCH faster than gray pixels (which can be demonstrated with my kindle video player or my "newtrix" demo. The fast update can be followed by a slower update that supports gray pixels, which can fade in without any annoying flash (depending on display mode). You can see that when turning pages in a book in some non-flash modes.
The tradeoff is that the faster you update, the more ghosting you get.
Many eink displays only use 80-percent contrast ratio (black and white pixels only have 80-percent of their respective color particles affixed to the front display surface of the eink beads). I did a demo (unpublished) on my K3 (my first kindle) back when I first got it, which forced 100-percent contrast ratio by redrawing each display line many times in pure black and pure white very quickly, followed by line of image. Comparing what I drew that way compared to other pure black and white drawn elsewhere on the screen made a HUGE difference (making "normal" black and white look LESS than 80-percent contrast).
I was always planning to make my native mode framework (a work-in-progress) support this GMOD (GeekMaster OverDrive) display mode for when you want to redraw the screen with "real" 100-percent contrast instead of the much-lesser-contrast "normal" black and white. Some day
RSN...