I think a great way to go with colors is to get a black vest and black boots, and dress it up color-wise with your t-shirt, or a helmet cover, or some electrical/duck tape on the boots. I say this because when I was 15 I decided that neon orange was a great way to go :no: and bought a bunch of neon orange and royal blue, and ended up hating it. Now I wear mostly black with a few touches of teal (horse wears a black bonnet with teal stripe, black boots with two skinny strips of teal duct tape, and I wear a teal athletic shirt with a black vest and a black helmet with a teal rubber band). So, if I ever decide I hate teal, or I want to nix the color altogether, I haven’t invested a ton of money in colored things.
All that to say, if you get your expensive stuff (mostly your vest) in a good neutral color like black or navy, you can add in touches of color with inexpensive stuff (like t-shirts, tape, vest laces, helmet cover) you can swap out later if you want to change colors or change where you add color. I think the countervailing opinion is that the charles owen body protectors are the safest (??), but some people find the Tipperary vests more comfortable. With the tipperary, you can get a black vest and order different colored laces for something like $8.
Color is great, but less is more; just make sure it all matches! (keep in mind, those aren’t actually rules, just fashion recommendations! you do you!)
I apologize to all the tack store owners for what I’m about to say, but when I bought my vest, I got fitted at a tack shop to figure out my size and then found one on ebay. But I’m on a pretty strict budget. But it’s a good way to get something safe and save some money.
Woof boots are a great brand for something affordable and durable (http://www.bitofbritain.com/The-All-Around-Boot-by-Woof-Wear-p/0035.htm). A good pair of no-turn overreach bell boots are important also. These fit into the back of the horse’s heels to keep the thickest part in the back to protect the horse from overreaching (http://www.bitofbritain.com/Nunn-Finer-American-Style-Bell-Boots-p/3322.htm). Lots of tack stores have all this stuff, so links are just for reference.
Affordable breastplates are easy to find at most tack stores, just make sure the fit is good, obviously. Cross country equipment gets pretty beat up, so I tend to choose things that are safe, but affordable, as it’s probably going to need replaced sooner than most tack.
Don’t forget a medical armband! If it’s a starter event, it’s unrecognized, so I don’t know what the rules are, but better to have one and not need it than the other way around. I personally wear a TaggCode bracelet instead–the medical info is more comprehensive and you’d be amazed with the armbands how much sweat your arm generates under that big hunk of plastic strapped to your arm. I find the small TaggCode bracelet much more comfortable and less distracting. And, they’re great for any other active thing you do, like running or hiking or biking.
Definitely ask your trainer about this, because you sound like you have a good relationship, but I put studs in my horse’s shoes every time I ride on grass. I had a horse fall down at novice, and actually saw a little pony fall this weekend at BN. It just always puts my mind at ease, and you can put in something really small depending on the weather. It’s likely at starter you won’t be making a lot of quick, tight turns, but my fall actually happened when I had a refusal and had to circle, so you never know. If you started in hunters, I’m not sure how familiar you are, but feel free to PM me for more information. Bit of Britain’s website lists all their studs by photo and tells you what each one is used for (http://www.bitofbritain.com/Studs-p/10112.htm). This is important to note early, as your farrier will have to drill holes in your horse’s shoes for the studs to screw into. Some might think I’m crazy for suggesting this at starter level, but again, better to have some small studs in and not really need them than to risk a silly slip that ruins your weekend. Again, talk to your trainer and follow her advice 
Have so much fun! I’m sure you’ll be hooked after your first xc run!
