"Gray out" used links

I would like to change the link color to gray after it has been used. I can do this by staying on the same page and using pop-up windows. However, when I have to go to another page and come back to the original page, I am stumped. I would appreciate your help. I would like to change the link color to gray after it has been used. I can do this by staying on the same page and using pop-up windows. However, when I have to go to another page and come back to the original page, I am stumped. I would appreciate your help.

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