Terrain tiles ....

[Galdar]

Why does some terrain tiles "go" green when you try to place them over a sloping terrain instead of on a flat ?

[Hitman]

If I understand you correctly, the problem lies in the tiledata.mul. Each terrain tile has a reference to a texture (from texmaps.mul). Whenever a tile is to be stretched (i.e. non-flat) that texture is used. So maybe there are invalid references ...
Btw.: If a terrain tile has no texture ID assigned, it will always be flat, even if surrounding terrain has different altitude (for example water).