the perfect kelvin

I'm wondering if I could use 5,000k cfl for veg and 3,500k for flower?

No reason why you can't, I've grown using them before

Would 2,700 and 6,500 be better?

I think the perfect kelvin for growing is 10000k but it is dangerous to use making it not perfect. 5000k is good to look at and i'm sure it's good for growing but you'll spend the extra bucks when you can just get a 6500k and it'll work fine.

I use mixed lighting for veg and flowering.

My vegging is usually

2x 65w 6500k Cfl
2x 43w 6500k
2x 13w 2700k
2x 13w 6500k

And flower

2x 43w 6500k lower on the plant
1x 250w 2100k bulb & ballast
2x 27w 2700k

Its perfect for me, I can maintain all the proper heat and humidity I need. 1 solo fan blowing from ground up and I get an Oz each dried... or more... for them only being 19" tall.

try to stay away from the 5000k as they throw to much green. i used them and wondered why my plants reached for my 2700k. one day while learning about leds i found out why. needless to say the 5000k are now in my bathroom and not in my grow box.