There is no problem with using ANY plant or animal oil.
Cod liver oil is also very good and was originally where the very romantic smell of leather comes from. Try it - I did after one of the leather chemists at the tanneries told it to me. I soaked a large piece of leather in cod liver oil and the fishy smell lasted about three hours. Then it changed into a pure leather smell. The very good Brittish conditioner, Dubbin, is made with cod liver oil.
On their forum, the leather chemists were unanimous: animal/plant oils were not bad for leather; mineral oils would deteriorate leather over time. In fact, they went further (might have been in a different discussion) to say that acidic is good for leather (pH of 4 is ideal). Leather is actually acidic as it comes out of the tannery: place pot metal on wet leather for 90 seconds and see what happens.
So, I really don't think there is a major difference between using neatsfoot oil, cod liver oil and olive oil.
Coloring? I have never mixed oils and dyes, so that one someone else can give you an answer....