There are a number of products that will promise to provide a "key", ESP by Bedeck and Zinsser BIN being the first two that spring to mind.
The former is water based, that latter is a pigmented shellac suspended in alcohol.
Either will provide a decent base to paint over, but neither will quite live up to their promises. By that, I mean that they will not as be as durable as they say they will be. Provided that you don't hit the edges with your vacuum cleaner, you will probably be ok with either class of product.
BIN, as a alcohol based product might be more difficult to apply consistently, but it cures much faster. 3 days to reach full cure Vs 5 to 6 days for water based paint. Additionally, when used over tiles, at this time of year, you may only need to wait an hour before applying the water based finish of choice. With waterbased primer such as ESP, you may need to wait until the following day before applying the finish coats.
Regardless of which you go for, ensure that the tiles are clean before applying any finish.
Oh, if you go for BIN, spend £2 on a bottle of household ammonia to clean the brush/roller. The stuff stinks but will leave the brush as good as new.