As an elementary art teacher, I have a huge pile up of markers that my little ones have left the caps off of and let dry out. We lovingly call these icky markers. I hate to just throw them away, so one of the things I do with them is make liquid watercolors.
Step 1: Sort
Sort your old markers by color. I usually have my students do this. They inexplicably love to do it. I also have them remove any leftover lids. We save those as an alternative art material.
The colors don't need to be a perfect match. There are lots of "in between" colors like turquoise that we mix with the blues or the greens. You could even make your own mixes by making piles of two different colors. Red and yellow would mix to make orange for example.
Step 2: Soak
Place the markers, tip down, in cups with a small amount of water in them. The longer the markers soak the more saturated the color.
In the picture, you can see on the left is a cup that has just been set out and is faintly yellow. On the right are markers that have been soaking overnight and are more saturated. If you want an even more saturated color, let the cups of watercolor sit out and evaporate for a few days.
I find that a couple days is about the saturation point of the water, so any longer than that isn't necessary.You can, however, continue to soak the markers in new cups until the tips turn white. A white tip indicates a fully drained marker.
Step 3: Paint
Remove the markers and paint. Liquid watercolors are great for painting traditionally, but are particularly useful for painting with eye droppers or spray bottles.
Step 4: Storage
To preserve leftover watercolors, funnel the paint into bottles. I didn't have a funnel, so i poked a hole in the side of a styrofoam cup. I also didn't have any fancy bottle, but after a mass email to my fellow teachers, I had enough empty water bottles to store each color.
These paints have a very long shelf life. They shouldn't ever go bad since they're basically just water and chemical coloring.