I have been looking for a white ink pen that will go down opaque over a dark black forever and I finally found it with the Uniball Signo. I was carrying around a brush and a container of Dr. Martins ink, but doing so is just not practical for sketching on location in a quick and spur of the moment fashion. The Uniball pen has solved this problem. You don't have to shake the pen or do anything weird to get it working and it goes down in much the same way that white out does. The consistency is close to a very non-viscous ink that would come from a fountain pen and has the staying power of "liquid paper".
On major problem I have found in the past with these types of white pens is that they dry very yellow. This pen does mute out warm but stays closer to the paper white than any I have tried before. Drawing over top with black after it has been put down is not a problem either. The ink is slightly raised from the surface of the paper so you may get a little residual on whatever you are using for your black, but it comes right off and doesn't ruin your other drawing utensils.
Hands down, this is a great pen.