I hope my 'explanation' made sense. The whole point is that
the 'gross' issues (controlled by timing, synchs, overall
deflection circuits) can't practically be controlled well enough
to make a (mask) color TV work. By making color entirely (well,
mostly) dependent on what appears to be a local (at the
mask/phosphor plane) off-axis deflection , which conveniently
is controlled by another 'gross' parameter (R,G B gun positions)
the whole thing becomes manufacturable/settable.
Purity controls have to do with making small adjustments
to the (e.g local 'red' deflection angle).
If you undersatnd all this you will see why the transition from
tubes that had the face as roughly the surface of a sphere centered
at the 'guns', to modern short/flat tubes took several decades.

Monochrome tubes in general don't have masks.

R.M.M.