If you have left your pen in the fused position too long either lying on it's side or upside down you may have some of the color seep into the mixing chamber nib... DONT PANIC! This can easily be fixed by putting the pen back in the fusing position and standing it upright so the mixing chamber is on top and the toning medium is flowing down towards the colored pen.
The natural wicking action should pull the color out of the nib.. depending on how long you left it to be contaminated it could take up to 5 min to clear the nib.
Once the nib is clear just color the excess toning medium out of the colored pen and Voila you should be good to go!
If there is staining on your Mixing tip you can also try using the Colorless Blender to help further pull that out.
In extreme circumstances you may want to replace the mixing chamber nib and you can get replacement ones at store.chameleonpens.com but the nib should still function fine if it is stained.. it just won't look as nice :-(
Just remember to not leave pens in fused position when you are not using them and try to keep them upright (mixing chamber on top) when you are fusing them. .. but if you forget its no big deal if you follow the steps above you should have not problem clearing the mixing nib.