I'm a novice gardener and have received a cumquat tree. The leaves have now (July) started turning a bit yellow. Is that normal? The tree is producing lots of fruit and they are ripening well.
Yellowing leaves generally means a lack of nitrogen or iron.
Apply a citrus fertiliser (which is high in nitrogen) - sprinkle it around the drip zone so as to not get too close to the trunk. You won't see instant results - but it will come better, especially when the new spring growth comes through (soon).
However it could also be an iron deficiency. In which case get some Chelated Iron and rake it gently into the soil - taking care not to disturb fine roots close to the surface.
After fertilising or application of tonic, it is good to water it in well with a fine spray so as not to disturb the surface too much.