It's easy to paint the narrow wood muntins between window panes. The hard part comes when you have to scrape off the dried paint from the glass. Here's how the pros do it:
First, use a sharp utility knife to score the paint on the glass. However, don't press the knife tip right up against the muntin. Instead, score the line at least 1/16 inch onto the glass pane. This technique eliminates the likelihood of your cutting into the wood muntin and, more important, allows the paint to form an air-tight seal between the glass and wood.
Next, use a razor-blade paint scraper to remove the excess paint. Push the blade into the paint, as illustrated, being careful not to scrape beyond the scored line.