Almost always **Unified Color**, which Blender turns on by default from 4.3. In that state Blender paints with `unified_paint_settings.color` and ignores `brush.color` entirely. VCK writes both and the banner shows you whichever one is authoritative, with a `Unified Colour (in use)` indicator. If they ever drift out of step, press the refresh (`↻`) button next to the swatch to re-sync.

No. The `RGB (Color)` session writes red, green and blue together and leaves alpha untouched, precisely so a mask packed into A survives a colour repaint.

VCK gives the native brush a proxy. The channel is expanded into a greyscale scratch attribute which becomes the active colour attribute; you paint that with every native feature intact — pressure, falloff, symmetry, blur, brush assets — and **Apply** merges the result back into that one channel. R, G, B and A are never read or written except the one you picked.

Blender tells you why if you hover over one. The usual reasons: - there is no active mesh object, - the mesh has no colour attribute — create one first, - a channel paint session is still running. Apply or Discard it first.

Yes — the operators are registered with Blender's undo system, so `Ctrl+Z` reverses a Fill, Smooth, Generate or channel transfer normally.

No. This is a hard guarantee. No material slot is added or removed, and no face's `material_index` is ever written. Previewing a channel is not a shader trick — VCK writes a temporary greyscale attribute and switches the viewport's shading mode instead.

4.2 and newer, including 5.x. It ships as an extension, so 4.2+ can install it directly from the zip.