The thing to remember about RAG status indicators is that 1) they're not objective, 2) they're different for every project, and 3) they're different for every company.
Before you set up the status report you need to find out what those indicators mean to your client. For some clients a 10% deviation from plan is Amber, and others it may be Red. It's going to be based quite a bit on their particular risk tolerance. The more significant the project is in terms of how it fits into their company, the more risk averse they're going to be, so they're indicators will trigger sooner.
As the PM, you're going to need to discuss this with them and find their tolerance levels to set up the status basis. From there, as you approach a change in status you'll need to keep their view in mind, and temper it with your experience to reassure and inform them.