One commenter asked if I'd seen the ODO links, and handily linked them.
Neither of them tells me what the difference is between ink and paint; nor do they tell me why we have two words for pretty much the same things.
So, this is a neat question that could have a deep answer, but you've closed it with unhelpful links that don't answer the question.
The closing comment mentions that it can be answered by a reference work designed to answer that type of question. Since one of the points of SE is to provide these answers it might be a good idea for you to link to one of these reference sites before you insta-close a question. And then, if you don't find the answer in any of the sites you look at you can change your mind and re-open the question later.
Would the question be re-opened if it asked "Why do we still use ink and paint when they mean the same thing, or don't they mean the same thing and people are just using them wrong?"