Hello, Happs!

You can touch up the red paint with whatever you think matches the color. Because it's a frame, not furniture, it's not likely to get much bumping or handling.

What about model paint? You can buy this in tiny bottles of high-gloss, in a zillion different shades.

I know those metal frames. They are easy to dissemble. Is the new poster valuable? If so, don't try to mount it on poster board, it will decrease its value. You might also end up with ugly bubbles.

Instead, try this. Once the frame is dissembled, lay the new poster down on top of the old poster, and put the glass on top. Most likely, the pressure of the glass will hold the new poster in place when you reassemble the frame.

Let us know how it goes!

You can certainly use a permanent marker or enamel craft paint, but they won't have the shiny finish effect. As mclarke suggests, you might try enamel model paint, as that might be shinier. You will have to experiment with colors, tho, which you might want to do on the back side of the frame.

If you are going to use the same frame for the new poster, consider having the glass replaced with museum glass. Not that much more expensive, but it will reduce the fading issues.