Neutral tint means the color does not have a dominant shade. The purpose of using neutral tint is to make bright pure colors darker without altering them and without turning them into mud. When mixed with pure reds, blues, and yellows, neutral tint makes them deeper and darker while allowing them to continue to sing.
Neutral Tint looks similar to grays. Why does it make bright pure colors deeper and keeps them pure and vibrant and black/gray pigments turn these colors into mud? It's because neutral tint is made of primary colors (red, yellow, and blue) mixed together - there is no black or gray in it! So, using neutral tint, you are only using bright pure primary colors!
I use neutral tint in all of my paintings. Sometimes, I pre-mix it on the side of my mixing tray/plate, but most often I create it by recycling paint on the mixing tray after I mixed all mid-tones, which is not very convenient.
Having neutral tint separate and ready to use can help save time pre-mixing and ensure the right proportions of red-yellow-blue each time. It's especially helpful for fast (e.g. plein air) painting when you need to make quick mixes and decisions before everything dries.
This Neutral Tint is a combination of Cadmium Red, Cadmium Yellow, and Cobalt Blue.