FEATURE REQUEST: Add colour options for neutral

[samgermain]

I don't want to fill my mask with grey, I want to fill it with white. Giving users an option for what colour to use seems like an easy addition

[samgermain]

I added my fix by changing lines 260-265 inside nodes.py from

if fill == "neutral":
            m = (1.0 - alpha).squeeze(1)
            for i in range(3):
                image[:, :, :, i] -= 0.5
                image[:, :, :, i] *= m
                image[:, :, :, i] += 0.5

to

if fill == "neutral":
            m = (1.0 - alpha).squeeze(1)
            for i in range(3):
                image[:, :, :, i] -= 1
                image[:, :, :, i] *= m
                image[:, :, :, i] += 1

but this would still be a nice feature

[Acly]

What did you need it for?

Adding a single "grayness" parameter would be fine. Not sure about adding r, g, b unless it's directly useful for something.

The neutral mode is more of a convenience thing anyway, since you can achieve same result with core nodes (some combination of solid color + alpha blending) with much greater flexibility. So it would be good to keep it purpose-driven.