Prepare the sight by cleaning oil etc off with thinner and allow to thoroughly dry.
Thin enamel paint to a water like consistancy. Apply with a toothpick or bamboo skewer tip or fine point brush by touching the rollmark and letting capiliary action wick paint into the groove. Touch multiple points as needed to fill the design. The less excess the better but it is not the end of the world if you get some outside the lines.
Let it dry 5 minutes & then examine. If the groove is not full enough (from shrinkage in drying) then repeat the application step as needed. I think the results look better with a fuller groove.
After 5 minutes dry time following last application use a patch dampened (not wet!) with clp or similar and rub across the rollmark to gradually remove excess. The more accurate you are in paint application the less you need to clean up.
You don't have to buy special paint. I've used flat gray primer sprayed into a disposable container and thinned with mineral spirits and gotten good results.