If you are looking for a near exact colormatch for matterhorn white it is GM pure white 12. ( a really old GM paint code)
I did a internet search and someone mentioned it was a match so I tried it out and yes it is an exact match.
I found that Duplicolor makes it
Duplicolor perfect match paint code is 12wa5111,522 available at Cdn Tire etc.
I needed to touch up my wheel pants and all the whites I tried were warm tint (creamy) or too bright whereas matterhorn has a blue hue in it.