Coloured visors might look a bit trick (to some) but they do tint the colour of things you see through them. For example a blue tint visor will make red lights look purple etc.
Plain iridium (or carra coated visors) are basically just a dark tint visor with iridium coating. The irridium coating generally doesn't distort colour too much (it can even improve contrasts) but it does reduce light passing through the visor. This means that an iridium visor will appear to have a heavier tint than the tinted visor it was made from.
Clear visors can obviously be a problem in bright sunlight conditions or where there's significant road glare. NB: If driving in those conditions without a way of reducing the light intensity wasn't considerred dangerous then cars wouldn't have sun visors. To demand that riders aren't afford a similar level of protection in those situations can be patently dangerous.
Tinted visors are legal up to a maximum of 15% light reduction but opthologists have gone on record and stated that in some circumstances up to 80% light reduction visors could be beneficial. The police however are perfectly willing to cause riders deaths by blind adherence to their own interpretation of some words in statute law books. Many people who ride with heavy tinted visors also carry a clear visor and that will often stop a cop form interpretating visor legislation wordings like a moron. That said however in some parts of scotland a few years ago police were confiscating dark tinted visors when used on bright sunny days. Possibly this was a simple power trip by the local boys in blue in that part of the country.
The simple fact is it's your life and you make your choices. Riding in poor lighting conditions with a heavy tinted visor could put your life and other road users lifes in danger.