Why PWM? Are these serial pixel LEDs, like WS2812B or similar, or individual-pin-multicolour LEDs?
If WS2812B or similar, you need a pin per string, OR you need to wire them all in series and run them from one string.
How many items total?
However, how are you talking to the tiny from the Arduino? Serial? Then you need to do the math about how often you intend to make changes to the LEDs. Occasional, sure. Fancy waves of varying colours at eyepopping speeds, don't bother to try.