If you do a search in the gunsmithing subforum you'll find plenty of threads on painting sights.
I have a stainless gun, so what I do may or may not be suitable for a blued gun; the other threads should offer some guidance.
What I do is degrease the area to be painted. I've used acetone and denatured alcohol with good results.
Next, I use masking tape to cover up the areas I don't want painted. Ideally, only the part of your front sight that you see in the rear notch will be painted. Of course, some people will have differing opinions on that.
I use Testor's enamel paint, but others have used nail polish or appliance paint with good results. I've even read about people using paint meant for fishing lures. You don't need to spend a lot of money on paint.
I'm assuming you want a brightly-colored front sight. In that case, I'd suggest applying at least one base coat of white paint. The white base coat will help make the color coat look brighter. You may need a second or third coat, depending on how thin the paint is. This is just my opinion, but I like to let a full 24 hours pass between coats to make sure it's fully dry; I will admit to being a bit overly cautious at times.
Then apply the color of your choice. Fluorescent orange seems to be a common choice, and it's what I have on my 642-1 right now. Fluorescent green is another good option. Basically, whatever color you're interested in trying. Again, you may need more than one coat depending on the results you want.
In the past, I've applied clear coats over the paint. I used clear nail hardening paint. It works very well to protect the paint job from wear-and-tear. One downside I found is that, depending on what you use, you may get glare on the front sight under certain lighting conditions. I've had my front sight "wash out" when this happens. I left off the clear coat the last time I painted my sight, and doesn't wash out nearly as much.
The nice thing is that if you don't like your results, you can always start over and try something different.
Final tip: Using a toothpick to apply the paint can give you a little more control over where you apply it, particularly on a serrated front sight.
Hope that helps.