When Norah was two years old and just learning to color, I had to wrap her little hand around each crayon and place her tiny fingers on the tip just so. Then I had to squeeze her hand gently but firmly and guide her crayon around the page for a moment or two, coloring with her. This was painstaking and she didn't enjoy my guidance.
After a while, I'd let go and let her color by herself. She'd hold the crayon correctly for a little while, but then she'd switch her grip and go back to an incorrect hold. Depending on what I was doing at the time and so as not to take all the fun out of coloring, I'd let her color incorrectly for a moment or two, just enjoying putting marks on the paper.
But, eventually, I'd start the process over again. I'd place the crayon back in her hand the right way and hold it there while we colored together again. Then I'd let her color alone.
We did this from the beginning, every time she colored in her coloring books. It ended up being worth the effort. After six months, she was holding her crayon perfectly. She never even thought to hold it wrong. When the time came for her to switch to markers for art or pencils for school work, she never had trouble holding those correctly. She could focus on drawing what she wanted or learning her letters, because she'd been holding her crayons right for years.