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!)
OK, let's get started. Step one...supplies. You need a flat brush, a fan, a small detail brush and a couple of Filberts. A filbert is a rounded edge brush that lets you maneuver the paint easily without going outside the edges. Ultimately buy brushes that you like though, and make them work. Fans allow you to blend, for example. I use fans a lot and it's how I graduate color throughout my images. Everything I paint is by hand....no airbrushing or assistants. I want it to be an expression of me and nobody else. You also need an oil painting medium to mix with the oil paint to make it more fluid, and to speed or slow the drying process. Whichever you prefer. Go to the art store and don't be afraid to ask for help....they'll love helping you! A medium is merely an additive liquid which increases gloss, makes it flow easily, preserves the finish over time, keeps it from yellowing. I personally love Galkyd and Galkyd Lite. If that isn't available, buy a medium that looks like liquid amber and is kind of thick. Don't buy watery looking mediums....too hard to work with.
Plaster. New dry plaster in good condition, which is to be finished with a paint other than water paint, should be given a coat of primer-sealer and allowed to dry thoroughly before being inspected for uniformity of appearance. Variations in gloss and color differences in the case of tinted primers indicate whether or not the whole surface has been completely sealed. If not, a second coat of primer-sealer should be applied. If only a few "suction spots" are apparent, a second coat over these areas may be sufficient.
By far, the most important characteristic in determining whether you are Warm or Cool is your hair color. If you can't determine your exact eye or skin tone, then your hair color will provide the answer. Why? It frames your face!
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.
If paints have been allowed to stand and hard lumps or skin have formed, the skin or scum should be removed, after which the paint can be stirred and strained through screen wire or through one or two thicknesses of cheesecloth.
If a desired shade is not obtainable in custom-or ready-mixed paints, white paints may be tinted with colors-in-oil. To do this, mix the color-in-oil with a small amount of turpentine or mineral spirits and stir this into the white paint, a little at a time. If a blended color is desired, more than one color may be added, such as a chrome green and chrome yellow pigments to produce a lettuce green shade.