I see you are new here, I dont know how to solve your problem but I can give you a little advice. The things that you change are not .mdx or .mdl files, as those are model files. You are looking for strictly a color change, this is done by editing the texture. Textures are .blp files. I would do some checking around the 2d art tutorials section. I will see if I can figure out a way to solve your problem.
EDIT:I believe I have solved your problem. Take this file and in your map open the import manager. Import the file. Double click the file and put a check in the box that says Use custom path. Change the filepath to UI\feedback\human-manabar-fill.blp this should change the manabar from blue to red.