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...this is how I do it:

1. Finish your repaint of the frame with a 2 part epoxy clear coat. Let that cure for about a week.

2. Use a fine artists brush or a paint pen to fill the pantographing with whatever contrasting color you picked.

3. Let that sit in there about five minutes until it starts to thicken.

4. Take a blue shop towel (the disposable kind, similar to paper towels), moistened with mineral spirits. Wring it out so it's not dripping.
Wrap a single or double layer of it around something like a dowel for a curved stay cap like that, or just a straight edged wood scrap for flat.
With a single swiping action, wipe off anything that you painted on there that has gone outside the clean, machined edge of the pantograph.

5. This will sometimes still leave some streaking on the bottom edge of your swiping stroke. You can usually clean that up with another part of the same blue, spirits moistened towel, around your finger.

6. With the advantage of the epoxy clear coat protecting your color coat, you get as many tries as you need, until you get it right. It's important to get clean, sharp edges.