It's really hard to replicate the effects of age as is shown on your existing saltillo tile. Definitely clean and possibly lightly sand the existing tile. Then you'll need to stain the new tile. AquaMix offers some stain but it's trial/error til you find what matches. And it may take several coats. It's an art project basically and you won't find just 1 correct answer to this. Focus on getting the existing grout super clean. That will need to be matched up as well. Once it's all said and done, we recommend applying a topcoat sealer across the entire floor (old and new) to get a consistent finish. (Note: keep sealers chemically compatible so they "stick" where you need them to stick)

Best of luck!