Body shop suppliers & many parts stores with a paint division will mix custom colors of the SEM. Pints, quarts or gallons. Many have the ability to put it into aerosol cans for you, too.
I agree with all the others. SEM all the way. Check out their low luster clear paint as well. A local body shop supplier turned me on to it a few years ago. It'll add just the perfect amount of shine to what you're spraying and will also protect it from scratches.
I have always used the SEM sand free as a prep before applying the color coat. It helps the color to "bite" into the surface and makes for a much more durable surface. Prep is the key to a successful dye application.
I use dyes as a last resort. Usually just to freshen up the original color of the material. Radical color changes are not a good idea.
A '60 Lincoln drop-top I did. The seats are leather & the door panels are vinyl. No problem acquiring the proper colors of leather, but finding matched vinyl was near impossible.
So I bought vinyls in colors as close as I could find, then had dyes color-matched to the leather & sprayed the roll stock before any pieces were cut out.
For solvent base I've been using the SEM Colorcoat since 1983, we buy it bulk and mix our own colors. The SEM spray can stuff is junk IMO.
ColorBond solvent in aerosol cans is pretty good stuff, covers much better than the SEM spray cans and the factory colors are spot on.
For leather if it's high end or retail I use some water borne finishes, much more durable and feels better too.