Unknown. But it probably wouldn't be good.
Let's start with the basics. The MC1R gene is responsible for various levels of melatonin compounds within your body. Specifically, it makes the melanocortin 1 receptor in melanocytes, melanocytes being the cells that produce melatonin. When the receptor is triggered, eumelanin is produced. When it's not, pheomelanin is produced. Pheomelanin is generally considered to be the cause of lighter skin colors, as well as lighter eye colors and hair colors. Stop the gene, and you stop the protein. Stop the protein, and you stop eumelanin production.
Except now is when we run into problems. Just adjusting the MC1R gene won't do anything immediately, even if you found a way to adjust the gene without mucking over something else. This is because the melanocytes already exist, so you have to wait for the new batch of melanocytes to take over for this change to be expressed. How long does a melanocytes live for? About 3-5 years. This means that, gradually, over the course of five years, all the subjects melanin levels will be shifted and thus be gently eased into their new body colors. At least, that's the best case scenario.
But we don't get the best case scenario. We currently don't understand exactly how human pigmentation works, so while it's definitely likely that the subject's colors will get lighter, it would be unknown as to what exactly it gets lighter too. It's possible that all you'll trigger is albinoism. Not to mention that the MC1R gene is also linked to immune and inflammatory responses, unknown how, so there's also the potential to trigger a permanent inflammatory response by turning it off. Among other things, included but not limited to, death. Because the body is complicated and screwing with it always had repercussions.
And, lastly, I'm not even sure that turning it off will have the desired effect in melanocytes, because as I said, cells are complicated. I haven't taken an in-depth look at the mechanism MC1R uses, but there's a good chance that taking the trigger protein out won't produce the effect you'd desire. And, as stated in the above paragraph, that's only one of many unknown variables here. If that was the only one, I wouldn't mind going in for a closer look, but it isn't. Tl;dr - the body is complicated, and you likely won't get the result you want.