But you have only 5 levels available for each primary, meaning 0, 64, 128, 192, 255.

I would not do it that way. My way would be:

let the PCA generate 4 timing interrupts, 0,25ms, 0.5ms, 1ms, 2ms and 4ms. On each PCA interrupt decide for the 3 colors if they are to unblank. That way you get 32 levels of each color.

to further explain, if you have a color as 01010 it would unblank as follows each 8ms

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Erik