Kat, the white rain lilies grow wild here, but I had seen some yellow ones growing on the side of the freeway in Houston, but it was in a very busy area, where 288 crosses 610. You would be risking your life trying to stop a vehicle and walk up that incline to get them....although it did cross my mind. I had never seen them any where else, growing wild around here.
I noticed a corner of my yard, near the bayou had a bunch of yellow flowers, but I didn't even go investigate....we have a lot of weeds with yellow flowers...but when I finally did walk past there, I couldn't believe my eyes....it was a huge patch of yellow rain lilies! And shortly thereafter, some of the big pink ones like Mac mentioned came up in another area of the yard. I believe they do have their blooms at an angle. They just showed up....hadn't seen them before and I have been here about 16 years.
I saw some big ones growing on a back road...big pink ones...I think grandiflora. I don't know where they came from, and I meant to go back and dig them up but I haven't seen them since, so someone else may have gotten them.