Alternating between two LEDS using BJTs
Try this: -







simulate this circuit &ndash; Schematic created using CircuitLab

When Q1 is activated Q2 is disabled. When Q1 is deactivated by removing the input from the left of R4, R3 will acquire enough current thru D1 and R1 to turn on Q2 and activate D2.

Both LEDs can share the same current limiting resistor too but maybe I might put a resistor across D1 just to leak enough current into the base of Q2. This or use two LED current limiters - the jury is out on this - depends on supply rails to a certain extent.