If you do still want to do the three colours, you can choose LED's with somewhat similar brightness, but they don't have to be too close. They will have different forward voltages, so will need different resistor values anyway. You need a PCB that has a single resistor per LED, not ones that have one per group. Most of the better boards are like this, but there are one or two thst group the LED's.
Then you choose the one with largest forward voltage (probably white) and set up a couple using a variable resistors (trim pots) to adjust the brightness to a good level using the contacts in the PCB to supply the power. You could even install them in the board and hook up the trim pots to the resistor contacts. Then set up a couple of each of the other colours and adjust them so they appear to be the same brightness. Use a multimeter to read the trim pot resistance and choose the closest standard value resistance. Then you know what resistor value to install for each colour.
If there isn't a standard value close enough, you can often use 2 in parallel or series to make up the correct value, but it requires a bit of fiddly soldering to mount them.