There is a slight glow from the cathode itself...
Whatever colour your cables are, that is the colour they will glow.
The UV cathode WILL NOT light up dark areas of your case.
(Your case won't look purple for example)
So if you have yellow cables, they'll glow yellow.
Blue cables... Glow blue etc.
Cathodes should never be in line of sight from the window anyway IMHO - So you'll never even see the "purple" of them.
@FrostedNipple - A standard lightbulb gives out yellowy~white light.
It ALSO puts out orange and red... But there is so much white you don't notice it.
Same as the UV CC's... They have a slight purple tinge, but the amount of UV light being emitted is far greater. (You just can't see it)
EDIT - A bright LED Fan will overpower the light emitted from a UV Cold Cathode - So if you want your cables to glow correctly, I would only use UV reactive fans and/or black fans too.