Hi Leanne,
Your rhody does look a bit chlorotic (yellowing of leaves), but not too bad to me. An application of acid-loving plant fertilizer is the easiest likely remedy, though as with all plants pH can play a role.  Since rhododendrons prefer acid soils with pH 4.5-5.5 being a normal range for them, if you put it in soil that was limed to a degree that most other cultivated plants prefer ~6.5 pH, then it may be struggling because of that reason.  Doing a soil pH test is the only way to determine actual pH, so that's a best choice. If you are sure you have more neutral (non-acidic) soil because of previous liming, you can add iron sulfate as an amendment which will lower the pH.