Thanks for your question Sarah. The Senate ballot papers are white because it is harder to read text printed on red paper. The House of Representatives ballot paper is printed in a light green to distinguish it from the Senate ballot paper. This makes it simpler for voters to know which house they are voting for and which instructions to follow when filling out the ballot paper. The different colours also mean that once voters complete the 2 ballot papers, it is easier to identify which ballot paper is to be placed in the ballot box marked House of Representatives and which one goes in the Senate ballot box. As well, when officials from the Australian Electoral Commission empty the ballot boxes and begin to count the votes, they can easily see if any ballot papers have been mixed up.
Check out this fact sheet for more information about electing members of parliament.