did change colors after drilling/gluing in a tube? or after turning it down between centers? If it's a straight geen dye cast that means it's very translucent and the thinner the blank becomes (drilling, turning, etc.) the more light is allowed in it. That alone will change the overall color. If it just the raw untouched blank and it's changed color did you post cure in a toaster oven? I've had a purple turn blue before but could never figure out why. It was a blend of blue/red and was PURPLE when cast but after post curing it came out bright blue. Stumped me for months till I gave up trying.