for example I currently want to change the grunts skin color to a greyish or brown skin color like a orc from lord of the rings, how could I achieve that?
This falls very much under the category of art and so is not something trivial for people with little experience in that area. The idea would be to get a base skin, e.g. by extracting from WC3 CASC or downloaded from the skin section, and then changing the colours it in a program like GIMP, photoshop, paint.net or other 2D artwork programs. They should natively support DDS textures, but BLP might need conversion to a standard file format using a tool like BLPLab.
Once the texture is in the 2D artwork program then it is up to artistic skills to change its colour. In some cases this could be as easy as applying some effect or altering channel ratios to colour shift it. Other times it may require some manual touch-up if effects do not produce good results. I cannot really give more advice here as I am not a very good artists.
Lastly the texture is exported as a DDS with DXTC5 compression for use by Warcraft III. If using a legacy version of Warcraft III or needing better file size compression then BLP can be used using a third party tool like BLPLab but this requires the correct mipmap count be entered manually.
For very simple recolours it is possible to use the in-game vertex colouring system. This does have limitations in that it only works where colour is initially present and is applied uniformly to the entire model which can make for an unnatural or silly look.