Reply To: Add color to pages for print

If you want the rectangle to cover the entire page in the final printed version, then you'll need to take it out a little past the edge and add bleeds to your document (in Document Setup), so that the printer can be sure they're not going to leave a while line down one edge due to a slight misalignment when they come to trim the pages. Ask your printer what size bleeds they want; 3mm is standard in the UK and (I believe) 5mm is preferred in the US.

If you're not planning on printing the document, then I suspect it doesn't matter.