I did this once...I used a combination of orange/red/brown....I know...you're thinking BROWN??? lol I was worried that the orange/red was too vibrant, that it would turn out like a tangerine or something, rather than pumpkin, so I added a touch of brown and it mellowed it out a lot and really resembled the color of an actual pumkin well...the outside of a pumpkin at least. And I just added enough color until the batter looked a little darker than what I wanted it to be when done, 'cause it does fade a teensy bit. If you want the inside of the cake to look like the inside of a pumpkin, I'd use orange/yellow with a drop or two of brown. Good luck! Take pics and let us know how it turns out!