I have a ziplock bag in my craft room labeled BAD PENS.
When I come across a pen that's drying up, it goes into the bag. Then I use them to make watercolors. All you need to do is pull out the felt nib...
... and drop it into a shallow container of water. If you can't pull out the nib, like with the yellow Crayola pen at the bottom left, just put the pen into the water. With most pens, you'll see the color start to leach out right away. That violet one, second from the left, was a highlighter. I didn't know if it would behave the same way as standard markers. (Answer: No.)
You can paint with your watercolors right away, but for full intensity wait a day or so.
If the colors are too pale, just leave your watercolors sitting out uncovered for a day or two to allow some of the water to evaporate. This will concentrate the color. Happy painting!