I was wondering: what happens if I put the resistor on the cathode instead of using 3 resistors for each color anode?
1 Resistor:
3 Resistors:
I'm asking this because it's a pain to place one resistor for each anode. Is this the only solution?
Also, I'm intrigued by the fact that a lot of tutorials about LEDs talk about using a 220 ohm resistor. Why? Even for a red LED consuming 25 mA, the required resistor for running it on a 5V line(that's what Arduino provides, right?) is only 120 Ohms.
You don´t calculate good. For all colours must be used individual resistor. And a resistor value can´t be less than 200 ohms! 
what happens if I put the resistor on the cathode instead of using 3 resistors for each color anode?
Then it won't work. Once you put the red LED on then the others won't come on because the red LED clamps the voltage at a lower value than is needed to turn on the other colours.
Why? Even for a red LED consuming 25 mA, the required resistor for running it on a 5V line(that's what Arduino provides, right?) is only 120 Ohms.
Most LEDs work at 20mA not 25mA. Yes you can use a lower resistor but most LEDs are good enough at between 10 to 15mA
what happens if I put the resistor on the cathode instead of using 3 resistors for each color anode?
Then it won't work. Once you put the red LED on then the others won't come on because the red LED clamps the voltage at a lower value than is needed to turn on the other colours.
Well, that makes sense. Thanks for the response. It's a pain to work with so many resistors :(.
. Yes you can use a lower resistor but most LEDs are good enough at between 10 to 15mA
That's quite some news to me, I always calculated resistors for the 20mA-30mA range. Isn't 10 mA too low though? Won't the LED be too dim?
Isn't 10 mA too low though? Won't the LED be too dim?
No, the eye's response is logarithmic so it is much less than half as bright.
Most LEDs are only speced at 20mA so I think you are over driving them.
