For most leds witht the supply voltage at 5v any resistor from 220 to 1k ohms will work, and. If you have less than 18 leds total you wont need any transistors so long as you stay to one led per pin, or use a transistor for say a couple leds in parallel(or series with higher voltage)
why not add sounds too? A little speaker with a transistor and you can make some cool old 8bit type sounds to match the leds
The uno has a limited amount of pwm pins so you will only have so many dimmable led "channels" ,