I have a very alkaline garden so chaenomeles and such often look chlorotic. I use a sequestered iron solution. For a quick lift, a foliar spray is effective, although it doesn't do much for the leaves which are already chlorotic, it does induce much better new green growth. Ah, the reddish tints does look a bit like a magnesium deficiency or some other micro-nutrient. I generally do a little Epsom salt foliar feed later in the summer. Possibly time to do a bit of soil testing?

Iron chlorosis appearing on Ericaceous plants is typically the result of insufficiently acidic soil. The availability of iron ions is determined by soil pH - higher pH soils tie up iron and lower pH soils release it. Chelated iron or other iron supplements will help on a temporary basis but the long term solution is to alter soil pH to an appropriate level.

I'd agree that soil testing is probably advised - at the very least a pH test but a full nutrient analysis will be even more helpful. And don't just do a home test kit. This is really a professional laboratory process and you will not get accurate results from a home test. UMass is an excellent source for this: https://ag.umass.edu/services/soil-plant-nutrient-testing-laboratory

btw, a magnesium deficiency will present almost the same as iron chlorosis.