"Canvas" was actually the first word that came to mind! Then I thought about the literal metaphor and worried it might not be clear - [i]technically[/i], if we treat an image editing program as a direct analogy to ink-and-paint, the "canvas" would be almost synonymous with "image", since one paints on the canvas. But...
And that's exciting news! I agree that color is probably less-essential - though on the other hand, it might also easily be filed under "no good reason not to". I can think of scenarios where it might be nice having a primary-color canvas, but at the same time, they're not scenarios I envision myself ever being in. But shading would be critical, there are times where I almost find myself wanting to load GIMP, and that's an experience no human being should ever have to go through
(snip)
I think our workflows are actually pretty similar. After a while, though, I actually feel mild tendonitis in my right middle finger from all the scrollwheeling around.
(really unrelated, but given the workflow discussion, I feel I should also give a shoutout to toe_head2001's "Quadrilateral Correction", which is invaluable with scanned images)