(It’s important to point out that the music for any grade test will be printed in black and white, so pupils will need to have made the transition to reading notes without colours. Check out the Rainbow Guitar Black and White Tunebook #1 and #2).
Here is a list of some of the guitar syllabuses available in the UK, and a rough idea of where the Rainbow Guitar books fit in:
These start with a pre-Grade 1 test called the Preparatory Test. Pupils would be ready for this about halfway through Book 2. By the end of Book 2, pupils should have met all the musical elements they need to take the ABRSM Grade 1 test.
The ABRSM Music Medal tests are teacher-assessed, so they can be done during a regular instrumental lesson (the tests are video-ed and moderated to make sure the teacher is administering and assessing them correctly).
Music Medals are rather different in content and style from Grade tests, so it is hard to make exact equivalences. The Music Medal progression is through Copper and Bronze Medals (which are pre-Grade 1 level), Silver (very roughly Grade 1 level) Gold and Platinum (both more advanced than Grade 1).
In Rainbow Guitar terms, Book 1 takes the pupil very comfortably to Copper Medal level. The pupil is then ready for Bronze around the middle of Book 2, and Silver towards the end of Book 2.
These are designed for pupils learning acoustic guitar. They allow for plectrum as well as fingerstyle playing, and feature strummed chords from the outset.
In Rainbow Guitar Book 2, chord symbols are used throughout and pupils have the option of learning and practising these as they work through the book. Assuming they do this, they would be ready for RGT Initial Grade early in Book 2, and RGT Grade 1 around the end of Book 2.
As the name implies, these tests are teacher-administered and teacher-assessed, and MTB moderates them to ensure consistent standards (see comments about Music Medals above).
MTB has a separate syllabus for each of Classical Guitar and Plectrum Guitar. There are two pre-Grade 1 levels called Introductory and Higher. Pupils would be ready for Introductory by the end of Rainbow Guitar Book 1, Higher by the middle of Book 2 and Grade 1 by the end of Book 2.