Real world application has nothing to do with advertising clams. I assume you want to do this once. Forever Black is not forever. In the real world it fades out and has to be reapplied. It is a thick grey goop that looks pretty for a couple of weeks if your lucky. It contains no pigments to "dye" anything. If it did it would be black and there would be a warning to wear protective gear/gloves when applying.
In order to actually "dye" something you need a base product with pigments that will soak into the plastics and form a finish.
If you want a reasonably permanent fix for faded dash, remove your dash pad, repair any lifted or warped edges to the vinyl then refinish it with proper vinyl paint. There is no miracle spray, goo or liquid that can be spread on bumpers, dashes or side trims that lasts long term. That includes mayonnaise and peanut butter. `)
There are several Forever Black products. The black dye says to use gloves and have damp cloth ready to clean off any inadvertently painted surfaces because you "cannot get it off the paint when it dries". (Notably, it is sold by my primary P-38 part supplier Atlantic British - whose wholesale manager has been extremely knowledgeable and helpful.) I will try it on my back bumper and the rear exterior "D" panels (which are black texture vinyl) and let you know how long it lasts in the harsh Baja climate.