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.
First consider the size of the painting. The more impressive the subject, the higher the emotions it appeals to, the bigger it is. Religious, mythological paintings are often huge - their massive energy makes you shiver. It is pretty understandable with figurative paintings like Rafael's "The Sistine Madonna", and more subtle with color field paintings of Mark Rothko. People are often overwhelmed with religious tremor in the presence of his artwork, and the size factors in. Also, the subject often calls for larger canvases - battle scenes need space and cannot be fitted into a smaller painting, while some subjects will get lost unless depicted in a smaller size.
Alkyd is an old interior paint made newly popular by a change in solvent - a super-refined petroleum chemical that has almost no odor. It is not a water paint. You thin it and clean brushes with mineral spirits or turpentine, or, if you want to retain the odorless feature, with the new odorless solvent. (Ask the paint-store man for just that, odorless solvent).
We've devised a simple, full proof way to determine whether you are Light or Deep. Look at your overall complexion, eye and hair color. On a scale of 1 to 10, with 1 being the lightest and 10 being the deepest, determine where you fit. So, a 1 would be the palest Nordic complexion with light blond hair. A 10 would be the deepest ebony skin tone and dark or black hair. So, for example: most Mediterranean and Middle Easterners have dark eyes and deep skin and are, therefore, Deep. Most Northern Europeans have blonde hair and light eyes and are Light. Where do you fall? Give yourself a number. If you fall anywhere from 1 to 4 then you are a Light. If you are 5 or greater you are on the Deep side.
Malabar and Designers Guild offer some striking brights while the new Shades of Sanderson comprises 120 colours tailored to Sanderson's collections. If you are aiming for a more subtle backdrop that will flow through several rooms, it's wise to stick to neutral shades. Kevin McCloud's Elements of Colour for Fired Earth works especially well with our cool, northern light.
Mixing. Paste paints, such as aluminum, resin-emulsion, and lead-in-oil, should be stirred with a stiff paddle and reduced to painting consistency with the liquids recommended on the manufacturer's labels.
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.