Pimples are going slowly but leaving red marks behind

First of all, thanks to that person who suggested to increase my water intake I realised I wasn't even drinking 2 litres a day but now I've increased it to atleast 4 litres a day and noticed pimples fading away slowly without doing anything else and now only some of the pimple remaining.

Now I'm struggling with the redness that these fading pimples leave behind. I need suggestions on how to deal with this redness and ensure that the pimples don't come back. Any suggestions is highly appreciated...