ou will be able to Put one LED on while the other is off and vice versa. i put two Red LED's on while two green LED's are off, so it's kinda fun to try with, i modded this guy's 555 circuit
With the supply voltage at the minimum for a 555 (4.5V) then it might stop working when the battery runs down a little.
The green LEDs will be extremely dim (only 1.1mA). The red LEDs will be dim (only 2.5mA).