Prizewinner is a Cucurbita maxima. The standard pumpkin you are used to growing is a C. pepo. The yellow color is completely normal and not the result of cross pollination. A lot of C. maximas (but by no means all) start off a color other than green. Prizewinner pumpkins typically get very large (100 - 200 pounds). If you leave only one pumpkin per plant they will be bigger than if you let several grow on each plant. I don't know if you know what they will look like but their appearance is different than that of your standard orange pumpkin. Out of the big C. maxima pumpkins prizewinner, in my opinion, has the nicest shape and color.

Wow! Thank you SO much. I am so glad to know my pumpkins are normal and will continue to grow. Looks like I have to make sure they have room! You are so kind to provide such a thorough answer.

Hello,
thanks to the both of you for your post. I have never tried growing pumpkins before, but have always wanted to. Every year I buy a very large pumpkin for our front yard. last year I was unable to dispose of our pumpkin because I left it sit through some pretty cold weather, so we rolled it down to our back yard and left it winter there, not realizing that it would volunteer a pumpkin plant. We were very excited and have let it grow, but I couldn't not understand why it was yellow. Thank you for your post.

Thank you for your input. I, as well, have basketball size yellow pumpkins and you have answered my same question. Looking forward to the results