I definitely think your bun is okay! I just about had a heart attack when I noticed my buns pee was a different color but as I've been giving them veggies, I've noticed the difference in the color. For example, if I give them 3 cups (3 buns bonded) split into two salads a day then it's very very light yellow but if I give them all 3 cups at one time the next day their urine ranges from really dark yellow to almost an orangey red. It's perfectly normal for the color to fluctuate especially when giving new veggies or new amounts.
If you're still worried -- I'd call your rabbit savvy vet and let them explain it to you!
My vet will take questions I have over e-mail or telephone rather than having to make a vet appointment for no reason. I'm sure yours would be willing to set up this same system for you, especially if your new to the rabbit community!