GoForSmoke:
It depends entirely on how your code interprets stick input. Read the input as force/relative-movement instead of absolute position and center stick == stop where you are. That's how the keyboard nub mouse on my ancient Thinkpad works. I quit pushing it and the cursor stops, it doesn't go to screen center.
True enough, but to address this in code the input device's behavior and conditions within the car have to be considered. There's enough vibration or changes in speed or direction during normal driving to cause a roughly inch tall "thumb" joystick to move, possibly enough to cause unwanted input while using a freeway ramp, hitting a bump, etc... So for that type of input device there either needs to be a "dead zone" larger than motion not directly caused by the user, or a way to enable the input only when it is wanted (as I previously described).
Of course, not all input devices have this problem. Tiny joysticks like the Thinkpad nubs, or tumb slide sticks wouldn't have the mass to generate enough inertia to move during normal driving. Similarly your other two suggestions below, or a touch pad, would be a good way to avoid this concern all together.
GoForSmoke:
Using beam interrupt or capacitive sensors you wouldn't need a physical stick, just a small area to wiggle a finger in or over.
Headroom:
Assuming that you'll have "something" to change the brightness with, one more control, e.g. a rotary encoder is enough to change along a "spectrum".
Instead of the usual RGB color space use the HSV (Hue, Saturation, Value) model. Changing the Hue would change the color and Value the brightness. You'd have to clamp Saturation.
If you had a two axes or two dimensional input device, hue and value could each be assigned to a different axis. A touch pad might be an effective way to implement this.