I've found that Golden and Amsterdam have good options for CMYK process colors available as acrylic inks, and these work well within refillable acrylic markers

I've found that Golden and Amsterdam have good options for CMYK process colors available as acrylic inks, and these work well within refillable acrylic markers from Montana, Molotow, et al. One caveat to the acrylic inks for a CMYK process is that they need to be really thinned down with an acrylic medium in order to get the proper transparency to achieve red, green, and blue.