what you can do is when you run out of purple yarn (leave at around 4 inches of the end to be able to weave in later,) just start knitting the next row with the orange yarn (again, leave around 4 inches of the orange yarn also to weave in later.) you don't need to actually bind off/cast off (end your knitting) until you are actually at the end of the scarf. you will go back later and weave in the purple and orange tail on the wrong side of the work.
check out the videos for casting off here
hope this is helpful!
Sean (click for my blog)
and/or visit me on Ravelry (boyforpele13)
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