Here is a partial mtl file.  The values allow diffuse, specular etc.

 

Note the map name has no path, so must be in the same folder as the obj and mtl file when imported.

 

newmtl 4_InnerMouth
Ns 100
Ka 0 0 0
Kd 1 1 1
Ks 0.501961 0.501961 0.501961
map_Kd V4SmplMouthM.jpg
newmtl 4_Gums
Ns 100
Ka 0 0 0
Kd 1 1 1
Ks 0 0 0
map_Kd V4SmplMouthM.jpg

 

try google for file specifications.