Hmm...

Did you carefully check the cables?

It takes only one pin to be malformed (either Monitor or Loop-Through Cable), in order to disable a Color Channel.

In rare Cases, one of the Pins might also be found 'retracted' somewhat, meaning it is possible for the normally fixed Pins to move inside the Connector housing, thus preventing contact. (can be solved by either carefully pulling it out again, or by opening the Connector casing if it already went too deep inside).

If none of it is the case, it could be a touchy/broken wire in either of the Cables itself (carefully move it around, or test with another Cable).

If none of this helps, you could actually test both the Monitor for failure (try on another machine, or try other Monitor), as well any possible damage on the Voodoo2 Card itself (check the Pin-Out going to the Connectors on the PCB, which could be damaged or short wired by being malformed).

Hmm  ??? Strange one this.

The only problem I had with Colour and a Voodoo2 was when I had some really old GLQuake patch for quake.  When GLQuake launched, everything was in blue, no green and red at all!  Blue Quake! 
But seeing that, that was a software problem....

Are you using that colour software in windows, it's called 3Deep.  It comes with the banshee.  If you are using that, it may have something to do with that.  If not, try getting a copy of it and installing it.  See what that does.  That's all the advice I can give on this matter. 

Before you can come to any conclusions on the card, the best thing would be to get a monitor with a good monitor cable.  Also acquire a decent stable pass-through cable.
Simply switching the monitor cable between the two cannot be considered as a long-term ideal.

Once you get the cables, you will be able to determine if there are any problems (if at all) with the cards. 

well... i don't think that switching cables was a problem... because on first time it didn't worked as it should be... i'll try third cable today... but interesting thing with the second cable... the first one, that was broken, had one pin missing... unused propably... second one had three pins missing... unused? it's working on every computer/monitor except mine... on board trio3d is sending synchronization signals on those two pins... third cable which i've got has got one pin missing... and i just can't get a cable with all pins... maybe voodoo is sending blue component on that missing pin? no, i don't believe it... but if it won't work, i'll just buy other card... i can't find anything above voodoo2 on pci... are those cards that rare? strange again... whatever, i'll propably buy voodoo2 sli;)

some pins are unused, right. maybe fasteddy's page will help you:
http://homepage.ntlworld.com/edwardbamford/V2/index.htm