Here's PPG's instruction on spraying velour per using there products but should work with any brand/equal product.
Enhancing Carpet/Velour
1. Clean with a good quality upholstery shampoo to remove excess soil.
2. Clean with SX1004 Plastic Cleaner and Prep. Scrub lightly with a nylon bristle
brush to remove oil, grease and contaminants. Allow surface to dry and then vacuum.
3. Apply 2 light coats of Elastomeric Color allowing 3-5 minutes flash time between
coats. Immediately after each coat, brush in all directions with a nylon brush to
keep the fibers separated. Use 20-25 psi for conventional equipment or 8-10 psi at
the cap for HVLP equipment.
4. Allow last coat to dry 5-10 minutes. Brush surface again in all directions to make
certain all fibers are separated, then vacuum.
The key point is to get the surface as clean as you can & keep in mind regardless of what brand you use, even if it's a direct factory match nothing is going to match 100% perfectly. So expect some variance in shades.