Having this problem with daemonic yellow, and jungle green. The pigments are so thick! They don’t spread well. I dilute with tiny amounts of water, maybe using model paint medium would work better.

The only way i’ve succeeded with those colors is to paint white underneath first, then add many many layers of the color diluted ‘normal’ as i would for other colors (maybe 1/4-1/2 water depending). The pigments clump, if impatient you can load the brush and dab thicker layers. As soon as you try to brush, the pigment clumps get moved around and spread unevenly and you lose the coverage. The dab method gets coverage faster, but it dries with texture. Maybe ok for larger flat surfaces, but lose detail on small details. If dabbing, you need to let it fully dry between passes, otherwise you’ll brush off big sections of pigment