It just turns orange as it ripens, which also shows when the pumpkin is ready to be picked. There isn't really a reason why vegetables/fruits turn colors. It is just the natural color they turn when they ripen.
brown
A pumpkin spider, or an orange orb-weaver spider.
a pumpkin is orange and a pumpkin while a shrub is not....
Pumpkin!
Because the pumpkin is huge
A pumpkin turns orange off the vine due to a process called carotenoid synthesis, where the plant produces pigments that give the pumpkin its orange color when exposed to sunlight.
pretty much the same only smaller and green
Since pumpkin is like orange, and orange and blue are complementary colors, the likely result is brown.
light orange (yellowy)
A orange melon that contains a lot of seeds
Pumpkin is a shade of orange that is commonly associated with pumpkins, it is also the name of an Anything Muppet pattern (the tall kind with the oval head)
Pumpkins are definitely orange. When you see one, you will know its a pumpkin!