Are you thinking local unaffiliated stuff or affiliated bigger county shows? I would try her in a hunter class first and you can ask the judge for their feedback at the end as to whether they think she would be better suited in a different class. Usually at the lower levels you won't find the different weights classes. In theory colour shouldn't make a difference however if there are 2 horses on a completely even keel the judge will pick the one they prefer which could boil down to colour. Sometimes it's good to be on something other than a dark bay lol! I love showing but always remind myself that it can be a case of personal preference at times. Does she jump? you may find working hunter could be a route to try? Good luck with whatever you try!
ETA you could always try and find a trainer who specialises in showing as they will be able to give you good advice to suitable classes and all the turnout requirements.
Your horse is lovely! Showing is hard because everyone has different opinions and it can be hard until you see the horse in person! I would enter a few local classes and see the feedback you get but I have a feeling she might have a bit too much bone for Large Riding Horse - from what I've seen they like them quite fine, pretty and flashy! But Middleweights is a good shout or Show Hunter if there isnt a weight class.
Then just ask the judge for some feedback - showing is a lovely day out and you get lots of lovely photos all dressed up... that is literally the extent of my knowledge and from looking around me at shows! LOL!