The LED's I supply with the kits are all warm white. They're the most commonly requested, and trying to have multiple colors really complicates a low volume operation like mine. 
If you really want bright white or yellow, I suggest the regulator modules and just buying the reels of LED's. For instance, I'm using red in a couple of the Hogwart's cars.
If you want marker lights or drumhead LEDs on your passenger cars, you can indeed add those. What I do is string them in series, then adjust the resistance in series to get the proper intensity balance between them and the strip lighting. It's somewhat of a balancing act as you'd probably either be using different color LED's and/or different numbers of LED's, so the resistor value is not a fixed and known quantity. For a single red LED, for instance, I've used a 2K resistor in series, that gives me about 5ma through the single LED and lights it plenty bright enough for the rear facing LED on the observation cars.
