They use 2 clear pieces then sandwich a color filter between. I have the main clear part. I might have some red filter too as I changed one of mine to amber, just not sure if it is long enough to make a complete red or not
Russell Jungwirth at Public Safety Surplus in Oshkosh WI had a handful of split red/blue Five-O domes for sale a little while ago. I've dealt with him and he's solid. Tell him Doug sent you.