Buyers need to know their standard not rely on cards from shows as an indication of quality. If they don't know the standard they should ask someone at their local club to help them acquire good birds.
Judges at Ag show are not going to disqualify anything for a colour fault. It normally attract a points deduction and the bird would just place lower. Some judges might decide not to card them if the bird was seriously lacking in breed characteristics,
but Ag shows get entries in from the wider community - kids, families and people not in poultry clubs. You can get all sorts of things entered. It's a friendly, low-key event that encourages community participation. The judge would like to see a line up of quality birds that can be awarded and when they come across a class of poor quality birds with no options their job gets challenging. The best bird entered in the class on the day wins though - even with some faults. On the weekend I entered some Sussex at an Ag show and they were in a class with other 'Sussex' with feathered shanks that were not disqualified. I'm glad they weren't. I saw a young boy of about 8 years old excitedly bringing his grandparents through the shed very loudly and proudly telling them about his birds. I hope he keeps showing and having a good time because the local club people there will help him out.
With regard to accreditation, this varies by state. Ag shows are usually where novice judges get their experience so you will often get non-accredited judges working at them. You will even get non-accredited judges working at other shows and events at times. They are chosen because of their known expertise in those cases.
The problems always seem to arise when someone says my birds that are for sale won their class at a show. People need to take no notice of that because it doesn't mean anything. If they won the whole show it might mean a little, but winning a class just means it might have been the only bird in it, or perhaps the best of half a dozen poor examples. It's not a recommendation on which you would buy something.
_________________Backyard Poultry Forum