A transistor can switch stuff on/off but LEDs aren't that simple. LEDs need current regulation ... and before you know it you've got a PCB full of components.
There's a much simpler way to do what you want: Use an integrated circuit, eg. the AN6884 chip which does switching, current regulation, etc. all in one little package.http://www.ebay.com/sch/i.html?_nkw=an6884
One of those chips can switch 5 strings of LEDs on/off with no extra components needed, and you can connect dozens of chips to a single Arduino pin (switch hundreds of LEDs with one Arduino pin!)
You've got 12V available so each AN6884 pin can drive multiple LEDs.
A white LED needs 3.6V so you have enough volts to connect three of them in series with a bit left over (3x3.6V=10.8V). Each AN6884 pin can therefore drive three white LEDs. Five pins per chip means each chip can light up fifteen white LEDs. Thirty LEDs only needs two of them.
Similarly... red LEDs usually need 2.2V so each AN6884 pin can drive (up to) five of them (5*2.2V=11V). Each AN6884 can therefore switch twenty five red LEDs on/off. Given that you have some extra capacity and red LEDs aren't as bright as white ones you maybe could put in forty red LEDs instead of thirty (ten strings of four LEDs).
Four AN6884s, two Arduino pins, the LEDs, that's all you need. No resistors, no transistors.
nb. You run the AN6884 at 5V, the same as the Arduino, and connect the Arduino's output pin to the AN6884 pin 7 ('AMP. output'), bypassing the internal amplifier (which isn't needed here).