Hi Miggy, No they cannot tell the difference between a black & white or colour TV.
They basically have a full version of the electrol roll and have those who have purchased a licence tiked off the list, the remainder they then assume do not have a licence, who are then sent a reminder and those who do not respond are those who are targeted.
There is a myth that TVL (TV Licence) Detector Vans did not have any detection equipment in them, which at the beginning was propably true but is no longer the situation. The likelyhood of eve seeing a TVL Van is fairly remote because the total fleet of these Vans only numbers about 30 for the whole country and some of them are "Dummy" vans that do not have equipment.
They usually target properties on foot using hand held equipment and then knock on the door. They will usually ask to come in to check, however they do not have any authority to do so. The onus is on the TV Licence people to prove that you have a TV and are watching TV on it, after all you may have a 50" 3D Plasma Smart TV and only watch DVD's and play Games on it, in which case you do not need a licence.
To get people to pay up they rely on threats of Court action which the say will end in fines and possibly prison, but they would have to prove that you were watching the the current TV Channels on your TV.