I'm not 100% sure, but I believe that (most) 4H showing is primarily about the kids learning about the animals and "showmanship" rather than the actual animal, where ADGA and other sanctioned shows are all about the specific animal. As such, non pure bred animals can be used in 4H shows, so a color differential shouldn't be a show stopper by any stretch. I may be wrong on this and your 4H group (leader(s)) should easily be able to confirm or deny this.
This can really vary a lot.
We have dairy goats so I don't know much about the market/boer goat/meat goat shows except the small one at our county which doesn't involve sale of the animals.
There are two basic types of shows in our area. Youth shows and open shows. Then- those can be sanctioned (by ADGA in our case, or I imagine ABGA in the case of boers) or not sanctioned.
Youth shows are only for youth. Many of the county shows are youth shows where you have to be in 4H or FFA. Most are not sanctioned. Rules vary. Some require registered/purebred animals, others don't. Some require record books and some don't. Some auction the market animals and some don't. Some are open only to residents of that county and others you can come from anywhere. Some pay premiums and some don't.
Open shows are open to anyone. Most, but not all, of these in our area for dairy goats are ADGA sanctioned. Many do have showmanship for the youth, but no FFA or 4F affiliation required.
Now, just to confuse: Our State Fair has a three day goat show. It's ADGA sanctioned. During that time there is a pay open show, no-pay open show, and a youth pay show. Also, record books, posters, judging contests, and showmanship for youth. So, my daughter shows each of her goats three times since she is a youth in 4H. Some adults come and they show their goats twice. It's crazy busy!
Our county fair is youth only from our county. Not sanctioned. Any "purebred" animal that meets breed standard can be shown. (One of seven dairy goat classes) There is also a market goat show that only require them to be under a certain age. No animals sold. They do showmanship, then the goat show where each goat is judged in the correct age/breed/ class.
With all these differences you really have to read the rules to YOUR fair an/or talk to someone who is familiar with them. We do 6-10 shows a year. This year we are done as of thisweeked (Florida has a very different schedule that most states due to weather) with seven shows.