In a circuit like this, having an even distribution of current among the LEDs (which also results in even brightness) is dependent on them having identical electrical characteristics. Red LEDs might have a forward voltage around 2v, while blue might be around 3.3v. Because the red LED requires a significantly lower voltage before it starts conducting, the electrons will take the easiest path through the red LED and the blue LED will never be lit. This also means that the red LED will have twice the rated current flowing through it because the circuit has only one current limiting resistor and depends on the LEDs to distribute the current evenly.
You may still notice differences in brightness levels when using several of the same color LEDs in this configuration due to manufacturing differences and other variables. Wiring the LEDs in series (unfortunately not possible with your 5v supply) or giving each parallel branch it's own resistor will even out the current distribution (when the LEDs are in series the current flowing through each of them will always be the same, as current must be equal throughout all points of a series circuit).
