Merge to an image the color from an other source.
Really simple to use, and the most accurate to preserve the luminance of the input “src”.
Mix Color: If the slider is on 0, the color is keep from the “Src”.
Mask: Use a mask like in all other nuke node.
(Un)Premult: To improve color correction of anti-aliased edges. This option is like in all nuke color node.