I checked the files again and the "usemtl ROOTS2" is included. I also changed the path to the new jpg-files in the mtl-file.
Still not working.

But anyway thank you so much for your help and tips [:D]

Hi, thanks for your reply.

I transformed every tif to jpg but still the same problem that colors are not imported. I now tried several files and I am confused because for some the colors are imported and for some they are not. But I can not figure out what the difference between these files are ...

I have the same issues with my files I downloaded from https://www.cgtrader.com. I am not able to find the exact link but here are some code-snippets (I hope they are helpful). Here are the first lines of my obj.file which has more than 19.000 lines and 13MB (it's a tree).
mtllib OC12_1.mtl ...