Help please

I have four hydrangeas in pots, two are large and lush, one is OK, but smaller, and the fourth is the same size as number three but yellowing.
Can anyone tell me what we should apply to bring the colour back to the one that is yellowing. There are two photos, the offender is the third from right in the photo of all of them, and it is the individual specimen in the other photo.
strangely, the light is showing the first one to look yellow, but it is definitely NOT yellowing, and it shows the 'sick' one to not look so bad.

https://trademe.tmcdn.co.nz/photoserver/full/655623433.jpg https://trademe.tmcdn.co.nz/photoserver/full/655622014.jpg

buy plant food for hydrangeas think acid

yes just checked my packet is acid

ok, thanks, I knew they needed acid not alkaline, but thought there might be another trick, 'somebody' always puts lots of blood and bone in and around everything, would that affect them. Should have asked that at the beginning

Looks ok really, a bit lacking in Magnesium.

There are many, many things to consider - some of them mentioned above. The first thing is that if they are in pots, the likelihood they are in a shrub or tub mix. Most of these mixes tend to be neutral pH to alkaline. Next issue - what hydrangeas do you have - are they blue, pink/red or white? Even blue hydrangeas grown in potting mix will likely first flower pink because of the alkalinity of the mix. It is only when aluminium sulphate is added (acid) that they will start to turn blue. Some hydrangeas react more to being in constant sunlight than others and being shrubs that prefer a cool root run in a moister, humus rich soil - being in pots that heat up isn't always conducive to good health. Also, overwatering can leach out nutrition and end up starving the plant of food.
The yellowing of the leaves, particularly if the veins are still showing greener, indicates iron chlorosis and a liquid iron feed will help quite quickly. Blood and bone and sheep pellets are nitrogenous fertilisers and will produce leaf but not necessarily flower or, indeed, compensate for the chlorosis.
The best thing you can do for all your plants is to give them the conditions they will thrive in - so good, regular feeds (that can also be foliar or liquid feeds) good mulching and regular inspection to be on top of any potential problem before it becomes a major one. Understanding what those requirements might be isn't necessarily covered in small print on the back of a label so a good all round general garden care book wouldn't go amiss as a point of reference. Either that or a Library membership!
So, going back to colours - Lime for pinks/reds, aluminium sulphate for blues. Some pinks can turn to blue and vice versa but others will not. Whites stay white no matter how hard you try. It's a question of knowing which varieties you have and what they do. Confused? No one said it was straightforward!

piquant.
three of them are in various shades of pink. I know they change colours with the fertiliser/additives that are used. I probably don't care if they go blue, although the original pink was lovely, and I WOULD like it to stay.
The fourth one is of a different type, and last year, when I bought it, was green with tinges of pink. Unfortunately I don't have a photo of it, although it could be showing in a photo of the area next to where it was sited.

so the first three are what I'd call mop tops, the green one is almost like a toadstool shape, wide at the bottom of the bunch, tapering almost to a point at the top. Does that make sense
my main hope is that they will actually flower! secondly is the problem of the yellowing and your mention of the veins still being green applies, so I will endeavour to get some liquid iron.

I have some spray-on magnesium that I originally bought as a method of relieving cramp, I have half a dozen bottles that I don't use now, as I'm using mag capsules. Would the spray that I've got be ok, I could spray it on leaves and the potting mix that it's growing in.
Usuaally in our house plants get too much fertiliser, better that that dead looking plants, but I know some prefer less fertiliser.
We also have some chook and sheep poo. (Haven't opened the bag yet, it might stink) Would that be of any use in this instance.

Epsom salts have magnesium and great for yellowing leaves - instructions for use on plants are on the bag.

Sounds like your fourth one is a paniculata type. Do you not keep the labels? Whilst they often do not give you a lot of information regarding the plant and its needs, at least, if we have a name, we can determine exactly what we are dealing with.
Some of the paniculata's grow quite large so it maybe that you won't be able to keep it in a pot for too long. But only you can determine which ones you have and what their individual attributes are.
As far as over-feeding - don't do that - it's as bad as not feeding at all. And also feeding in pots can be detrimental too - you can get a build up of salts. Probably best to annually remove the plant, root prune and totally replace mix, giving foliar or liquid feeds as necessary during the year.

I don't THINK it's a paniculata type.
as for keeping labels, it was at the supermarket, no name whatsoever, but I loved it's looks, so bought it.
I don't often keep labels, I recognise most plants by their appearance (I got 90%+ in a plant knowledge test many year ago) mind you, I've forgotten a lot of what I knew. At that time I had over 2 acres and could have told you the name of every plant, other than maybe some that had been given to me by friends who didn't know what they were.
Also, I will admit, I know the genus, but not always the variety.
Some are tricky too. take lysimachia. There is the tall one commonly known as gooses neck, and a yellowy-green ground cover that looks nothing like it's big sister. I know this plant is considered a weed by some, but I want it again in my garden, and what I bought on here last year as the gooses neck variety has turned out to be the groundcover one.

epsom salts

yep, on my shopping list, thanks, I just needed a reminder