My son has entered his first intro test which it says "may be led", probably won't know until the day whether he feels confident to ride on his own or will want me to lead him - but if I do what do I wear?
High heels, mini skirt.... JUST JOKING!
I haven't heard of any specific guidelines, but personally i would stick to darker colours so that you are not distracting from the pony and rider. The main thing i would focus on would be footwear for comfort, particularly if it is in an indoor school with a soft surface. I know that there are some 20m trot circles in that test and it can be quite hard running round a deep surface, so I would try to find out what the surface is like. (Of course, if you are fit then it won't be a problem so take this with a pinch of salt). You have probably already thought of it,b ut don't forget safety of toes if the pony steps sideways by mistake.
If an outdoor grass surface, then watch out for 'slippery when wet' areas too.
I hope you all have fun.
Whatever you already have that is clean and tidy will be fine.
I used to lead and side-walk in RDA dressage competitions (up to National Level) and we used to dress to match the rider: beige jodhs, black riding hats, navy/black/tweed jackets (all matching as a team), long or short riding boots to match rider, etc. If it were less formal I would consider a discreet but dark outfit such as polo shirt and dark trousers appropriate.
Make sure you have good boots on of course and be prepared it's a long way and on a surface too! The lead rein class at our local junior show, where I wrote for the judge was stunning! Turn out for horses, riders and leaders was amazing. They really made an effort. The smart ones swapped half way around, into A is a lot of trot! I am going this year just to watch the first ridden intro class, that was like watching a rodeo last year and boy those kids stuck to it!
Good luck I hope you do well.