Is there a way to do per flake colors with Substrate? For example with other renderers i was able to link a color ramp to the flake and it would go through the range of color in the ramp and each flake would have its on value. Some of the car paints i work with have a range of color in their flakes. The color range is not related to the viewing angle or light direction, its jut different colors of flake media blended together. I am new to using substance, so i may be missing something, and if so, sorry if there is a simple solution to this situation. 
You could use Substrate Add nodes to blend multiple slabs that have different colored/scaled glints. Basically the slab would only produce glints, no base color or coat layer. Add those on afterwards with different slabs.
So essentially: A slab that controls the base coat of paint, multiple slabs added together for the flakes (glints), and a slab to add the clear coat on top.
Just a warning though the material will be extremely heavy, but i’m guessing you’re doing visualization work if you’re looking into tweaking flake colors.
Would love to see what you come up with!