Indian artists employed multiple perspectives unlike their European counterparts in their paintings. The idea was to convey reality that existed beyond specific vantage point. Some of the special Miniature paintings include illustrated manuscripts of Jains and Buddhists, the flowering of the Mughal, Rajput and Deccan miniatures. Themes used were from Indian epics like Ramayana, Mahabharata, Bhagvata Purana, Rasikpriya, Rasamanjiri as well as ragas of Indian classical music, etc.
Like this, it will actually take quite a while to reach a number that is close enough to the yellow color you've chosen that you can't tell any difference (and mathematically, you will never actually reach an average of 4!)
What colors are prevailing - tonal or local, warm or cold?
Scuff for paint. Once your Mustang is totally stripped of exterior chrome it's time to scuff up the existing paint. The paint shop should do this also, but on the lower end paint packages they won't spend much time here. It's better for you to do it so you know it's done thoroughly.
Basically what you're looking for is a good, solid foundation of existing paint that is stable and still strongly bonded to the car. Many times an original factory paint job, even after decades will meet these criteria.
Take off the chrome molding around both the windshield and rear window. You can pick up a special tool at your local Mustang shop or by mail order that removes these quickly without scratching paint. Of course your paint is probably not in great shape at the moment, so a large flat screw driver will also do the trick. For some classic cars it makes sense to even remove the windshield, but classic Mustangs have chrome molding that will help cover any slight overspray.
Once the drywall compound/spackle has had sufficient time to dry, you will need to lightly sand the surface of the repair to smooth out any ridges that may have been caused while "coating" the patch. Once you have the surface sanded smooth, take a wet sponge or cloth and, by lightly washing/rubbing, blend the outside edge of the drywall compound/spackle back into the wall. For a smaller repair, you can wash away the excess drywall compound, leaving only the compound/spackle only in the area where the indent was filled. For a larger hole, after filling, sanding and using a wet sponge to blend the outside edge of the compound back into the wall, it may be necessary to texture the area in order to give the smooth patch the appearance of the existing wall.