[Evidlo]

If LEDs share a resistor then they dim as more LEDs are turned on.

[spuz]

I meant to say why does each LED require a different resistor value?

[dfox]

Because different colors usually mean different semiconductor material and thus different forward voltage of the junction which in practical terms leads to different brightness if the same resistor value is used.

[ilyt]

Different color have different forward voltage and luminosity per mA.

Or if it is connected to some live signal the signal itself might be with different voltage range