I remembered there being something about not doing the book out of order. This question was asked on their website and here is the response from Durell Dobbins who wrote the curriculum.
"Can we skip Year 1 and start with Year 2?The Rainbow curriculum is written from the perspective that an understanding of physics is fundamental. The program then builds on that foundation into a study of chemistry, which is an application of physics, and then into the study of biology. Because our program is structured to build on concepts, beginning with the Year 2 curriculum defeats the purpose. Also, our Year 2 program requires the use of many of the items from the Year 1 lab kit.+"