The casters I've spoken with have generally requested "materials specifications"; info on what plastic we want to use. Since the only lead I have is "resin", I've explained the requirements to them in plain english.

Here is my list. Please add, correct, or remove from this as necessary. That way, we can have a more accurate request.

Physically:
come in contact with skin
resist wear and color bleeding
be able to be created with and retain a slight texture
be sturdy enough to withstand years of use
survive reasonably extreme ambient environmental temperatures (perhaps -40 degrees Celcius up to 150 degrees Celcius)
strength for the thin wall of the buttons' shaft

Resin was successfully used in the past (for colored).
Could material used for the original pieces be determined by looking at them?

Color:
create sets in each of five different colored translucent plastics; red, green, blue, yellow, and a dark grey
be lit from behind with with small LEDs
match existing acrylic lenses color in daylight
when lit, match the color of the text on the display
colormatching process will take a few itterations of proofs to determine the correct colors

http://www.intelliwebplus.com/empegbuttons/
Shows the device, the buttons and knob as they appear lit and unlit, and the lenses to match.
The relevant colors on that page are blue, red, and "smoked" (which is the grey).