Reversing polarity of a load with PWM
simulate this circuit &ndash; Schematic created using CircuitLab

Figure 1. A simple NPN transistor inverter will allow driving the H-bridge from one micro-controller pin.


When CLK goes high A1 out will go positive and A2 to GND.
When CLK goes low A1 out will go low and A2 to V+.
By varying the duty cycle from 0 to 100% you can change the colour from 100% D2 to 100% D1.
Note that with this arrangement it is not possible to turn the LEDs off. You would need a second microcontroller output to switch the enable pin.