Take your marker completely apart. Take out both nibs. Now when you take the end of the marker off (so that you can access the ink barrel), it is very tough. I recommend taking the brush tip out first and then using a pair of pliers to kind of 'pop' it out. This is what you should have now.
Run the nibs and color barrel under water to get all of the ink out. The barrel will be white when you are done, but the nibs may still be stained. Make sure you 'write' all of the color out too. I just did this in my kitchen sink. I should have showed you with a darker marker, but even when you clean out a red marker all of the ink comes out.
Before refilling the nib with ink I kind of pinched the water out of the ink barrel. Don't pinch it too hard, but you should be able to squeeze out a good bit of water. Now just put the nibs back in and refill the barrel with your new ink color. This one is Pink Pirouette. I started filling the end where the fatter nib goes and filled the barrel most of the way, then added a bit to the other end. I also added a little bit of ink the fat nib its self.
Now just put your marker back together. I will make labels for mine so I can be sure which color is which! Voila! You now have your very own Stampin Up In Colors markers!!