Do diamonds change to a grey color?

I have a quad ring and one of the diamonds fell off and i took it to the jewler where i bought it. They fixed it polished it and cleaned it. They gave it to me and one of the diamonds was dark grey. I told them it wasnt like that the cleaned it again they said the diamond was fine it was cleaned thats why i never notice it the color i said i didnt buy that way and ive cleaned it before. Then they said i have a yellow stone but the other three are their original color. They said they never touched the one that change color. They dont know what happen to it please help any advice