For terrain, I always use house paint...doesn't really matter if it is interior or exterior. Where I come from, there are several main manufacturers available, all with online colour charts to select from, and you can also go to the hardware store or paint specialist shop and get colour swatches and also small sample pots. These paints always have a huge range of colours and shades available that you can select from.
...and because they can come in tins from 250ml up to 4ltr or more, you can get whatever size you want...I usually get 1ltr tins of the main base colour, and 250ml or 500ml tins of the stuff that will be used for highlighting.
...and because they give the colour mix ingredients on stickers attached to the tin, you can then very easily get the same mix made up years into the future to perfectly colour match any future projects you want.
I did some fantasy dungeon tiles several years ago, and used three different shades of grey (1 for the base colour, then 2 lighter shades for drybrush highlighting)...and I still have the tins with paint left...but if I didn't have any paint left, and I ever want to make some more dungeon tiles, I could easily take the mix details and get some more perfectly matched colours mixed up at any number of places close to my house...even phone ahead with the order, pay over the phone with credit card and have the paint delivered or just walk in to the shop and pick it up...