That's what cedar does. If you want color, you need to apply stain.

You can apply clear stain to stop the graying, but if it has already started you might not be able to go back, only keep it from getting worse.

Cedar naturally ages to a silver gray over time. If you want to maintain the natural wood look (seeing knots, etc.) you may need to pressure wash or sand the wood (depending on degree of aging) to avoid seeing the gray that is already there; you can then use a clear or semi-translucent/transparent stain and then re-apply over time. If you're not too concerned about seeing the knots and grain, you can use a heavier stain that will not show through, in which case don't worry about the gray, it should be hidden anyway. A good quality local painter who works with decks can take a look and help you choose a good stain/maintenance option.

Second what Harding Carpentry said. All products eventually fade to gray because of the sun's UV rays. That gray on the boards have to be sanded out or pressure washed off to get to the rich color Cedar originally had. You may have to repeat this process every 6 months toe very year depending on how much sun your deck gets.