Acorn is a 36" skewbald registered Shetland, so he is out of Mini but not upto height Standard, we have done lots of showing and been quite successful at local and county level, breed classes are slightly different, we do broken coloured classes where possible and as he is a gelding he ends up generally against the Mares , as gelding only classes can be far and few between.
He is now being ridden and once we get fully off the LR he should do better in the ridden classes.
Having a coloured gives us other options, CHAPs classes and coloured classes so we get more than the one breed class in other levels.
Much depends on what you want or how serious you are in showing, first decide on Mini or Standard and if you are looking in hand then I would go more towards 40" , Black generally covers browns, and coloured is everything but them, broken coloured = Piebald & Skewbald
Mare or Gelding is of choice - Mares do tend to come out well but thoughts are changing and Geldings are taken more seriously now.
Have a look on the SBSBS site and Shetland.com is a good site for seeing what is on the market and looking at the studs in your area, have a look at a few , different breeding gives different results - I love coloureds so I am a little biased, but maybe we should get a Black and then we would have every class covered, but I would put the limit on a stallion.
We have Nuts as a companion bought her at 18 now 20 and she has turned herself from a companion to a second show pony for the kids (when shes fit - a few health issues at the moment), she is chestnut so goes in the Coloured classes at Breed level but obviously not the Chaps, and we have veteran class option with her, we take them both so are busy through the day. although neither have been out this year thanks to injuries. First time in 7 years wall is not covered in Shetland Ribbons!
wouldn't worry too much about photo image markings, might make a difference in a coloured and markings class for Chaps but not at a county level breed show, its just a plus on viewing that's all.
A good pony is a good pony looking the same each side will not distract from that.