I guess I can use bit-modulation

Not only did I get too few 150-ohm resistors (I bought 30 of each, and I'll need 60 of the 150-ohm's.) I also decided to go with the bit-modulation anyway. The circuit I have in mind is essentially 3 banks of up to 30 LED's, so I can just multiplex between the red, green, and blue. I ordered several MIC5822 8-Bit Serial-Input Latched Drivers — they contain a cascadable shift register with an 8-bit latch and the outputs can sink up to 150 mA to ground.

Originally, I anticipated making a capacitor matrix that would keep the LED's from depleting and multiplexing the analog outputs on the PIC controller to charge the capacitors. It might have worked, but this bit-modulation system is way better — even if it flickers a little.