Ive got my terrain that I create and paint in runetime.
I want to add some color to it (RGBA), like a little bit of red with low alpha on some places, some blue over there, etc.
How can I achieve this? (please dont recommend me assets)
In the UnityEngine.TerrainData class, you can define the set of colors by setting TerrainData.splatPrototypes, where each SplatPrototype has a texture, which in your case will contain some solid RGB color.
You then generate a set of alphamaps (one for each SplatPrototype) to blend the different colors together. Once you have this set of alphamaps, call TerrainData.SetAlphamaps.