Pigeon shooting
It has been reasonably established that pigeons view the world in shades of grey, and whilst they may not be able to distinguish colour, they can most certainly recognise the human form and its associate dangers.

The purpose of camo clothing is to break the outline of the shooter, so it makes sense to try and replicate the shades of the background where you build your hide.

If the colours blend in naturally, it follows that what the pigeons see, whether in colour or not, will also blend in.

Of course, there is also a huge market in this type of clothing, and manufacturers will be keen to convince you their products are essential for success, but the best camo in the world will not be effective if you cannot keep still when pigeons approach.