Hi, Welcome to the forums!
There are alot of topics like this. LED tail light projects are popular. Do a quick google search or use the search button here and you will ind alot of ways this can be done.
For the 2 levels of brightness, you can simply use two differnt values of resistor (higher value for dimmer lights) or you can use PWM. If your not familiar with PWM, you can look that up here too. A simple 555 timer circuit will work well.
To turn your white LEDs on, you will have to hook them into the factory reverse lights that are controlled by a switch on the transmission.
You can use a toggle switch to simply change what set of leds are on. Either red or blue.
Beware blue tail lights will get you a ticket unless you just use them offroad or at shows.