Current questions and answers regarding the map editor.

I just want to turn Arantir into Gandalf wearing gray robes, and instead of summoning a dark creature, he summons an emerald dragon – that way I’ll get a kind of light mage.

Are you sure that simply recoloring Arantir will allow you to make him look like Gandalf? Although you could try using another hero for this purpose.
Modding section.
And here is a thread where Chopic dealt with a similar problem and even posted screenshots. However, he mostly plays around with specializations, but the model can be changed there as well (that is, in this case, change the texture of the model).
And here is an FAQ; it covers some basic questions, including how to recolor things.