I'm not an expert with meshes but this is what I've found regarding these three issues you have.
Yellow and black meshes: Black and yellow means the coding is all right, but the game can't find the actual texture picture, or it might be spelled wrong in the coding.
You should make sure you actually enter the correct material name for the meshes, and if you're really sure they are correct, it wouldn't hurt checking the .mat file for the meshes and possibly the textures it wants to use. If the names in the materialfile is written wrong or the actually texture picture is missing, this probably happens.
White meshes: Hello! There is an annoying problem. When you put in a new mesh, you need to close and re-load feralheart once you have downloaded the mesh. Some meshes have no textures, so they show up white.
Or
Find that material file and open it in Notepad, you're going to want to type in the name of the .png file that came with the mesh into the space after texture
Meshes crashing your game:
Go into C: >>> Feralheart >> media > particles and delete any ending in _0, _1, etc.
Change your rendering system in C: > Feralheart. Go to orge.cfg and replace the first line with either:
Render System=Direct3D9 Rendering Subsystem
[Direct3D9 Rendering Subsystem]
Or this:
Render System=OpenGL Rendering Subsystem
[OpenGL Rendering Subsystem]
It depends what rendering system you have. If you have the first one, change it to the second one. Vice versa.
Hope these help! Let me know how you go with it.
So glad you got everything sorted out! Thank you so much everyone for your help. I'll be locking this now~