Color Correction - Nuke 101: Professional Compositing and Visual Effects - page 200

This means whatever you now do in Grade1 will affect only where the radial's
branch is white.
9. Increase the whites by bringing the Whitepoint property for Grade1 down to
around 0.51.
10. Some of the deep blacks have become a little too gray, so decrease a little on the
Blackpoint property. I left mine at 0.022.
At this point, the grading is finished. Mask inputs can be very important in color correc-
tion because a lot of times you only want to color correct an area of the image. But re-
member not to confuse mask inputs with mattes or alpha channels. The use of the mask
input is solely to limit an effect—not to composite one image over another or copy an al-
pha channel across.