Make sure you have no connectivity between RGB lines and ground as well, except when the cable is connected to the TV (then you can expect ~75ohms to ground).
You can use the ATI Catalyst Control Center (or whatever they call it) utilities to adjust your colour temperature/intensity. Some cards will have different default colour output levels to other cards, however it can be adjusted to your eye. In particular with a 5XXX card of mine, I found the colours needed the "warmth" setting adjusted from around 6500k to about 7000k.
It may also be that your TV tube is old and tired. The RGB guns wear out over time. On the other hand, if you can access the TV service menu, you may be able to adjust colours there. Some TVs will have pots, probably on the neckboard, for colour gain and cutoff. Mostly you can leave the pots alone, but if all else fails.