Case: A appearance issue on a rear lamp part for automotive due to first use of a PMMA material. This is a blue opaque colored plastic including metallic particles in order to get a shiny aspect. There is important visual defect such as flow mark due to poor aggregation of particles with material.
This sounds more like the melt front losing injection pressure allowing the metallic particles to separate out or a knit line, which will be very difficult to solve. The melt fronts collide and then the particles fail to align in the same direction, creating a different look. Melt and mold temperatures will help, and valve gate changes as well, if used in the injection mold.
If you are adding the metallic content blending it at the press you may need to install a mix nozzle on the injection unit to get better dispersion. You alluded to flow marks. I have run red and clear PMMA for tail light lenses and both are prone to jetting from the gate. Start the injection slow for the first 10 -15mm then ramp up. Your mold needs to be well vented also, not just at end of fill.
We trialed metallic flake back in the day at Valeo Sylvania, in the Cadillac CTS bezel, forward lighting applications. We were never able to get the metallic flake to meld at the knit line. The smaller your flake, the hotter your tool, and the higher the available clamp tonnage and injection pressure the better it will look. However regardless of what we did the knit line area was always “dark”, even though we had no visible knit line. Are you even going to meet FMVSS requirements with a blue acrylic lens? Actually I’d try adding it to a hard coat application instead of molding it.