Back up a second. You said you have white and yellow LEDs, right?
Whites and blues run in the 5 to 6v range, all the other colors are half of that, down around 2.5 to 3v. Secondly, each is going to pull a certain amount of milliamps (mA). Find out what those are.
So now you've got three issues - white voltage, yellow voltage and total amperage. Since white is the larger of the two voltages, you need a power supply that will give you 6v and at least as much mA as whatever that total comes out to. Chances are you won't find the exact amount so make sure the adapter can supply MORE mA that what you need.
Pre-installed resistors on the LEDs may not be a total help since they assume you're starting with a certain supply voltage. If that assumed voltage isn't 6V then you need to replace the resistors with the right ones.
http://www.rapidtables.com/electric/resistor.htm
That should help you figure out what the values are of the resistors you have compared to what you need. Plug in the various amounts you do know and calculate for the rest.
Modular
Models
Build your fleet
YOUR way.
http://www.modular-models.com
----------------------------------------------------------
"I know you think you understand what you thought I said, but I'm not sure you realize that what you heard is not what I meant." - Alan Greenspan
____________________________________
"The customer that spends the least complains the most."