This is a photo that I volunteered to fix. There was many folds/dents in the photo, and I think I cleaned up most of the big ones, but my next step comes in. I would like to remove the yellowing (that represents the age of the photo) and have the color scheme follow the rest of the photos color. I don't have enough rep, but this is the photo I am working on.
https://i.sstatic.net/g7Vau.jpg
There are a number of ways to correct the yellowing in Photoshop. I think the easiest to try is to use a curves or levels adjustment layer, and use the grey dropper to select a neutral area. That should neutralise the yellow cast.
Below is a before image I artificially faded.
I selected the grey dropper (circled) and clicked on the cloth/paper under his left hand, which looking like it might be white or grey. You can see the blue has been raised, and since blue and yellow are complementary, that reduces the yellow in the image.
You could do the same thing with the Color Balance adjustment layer, moving the Yellow-Blue slider to the right. Below is the result, possibly a bit overdone, but the yellow is gone.
With that particular photo (and other, similar, photos), the easiest thing to do would be to use two Hue/Saturation adjustment layers. The first one would have everything else left alone, but have the saturation set to 0 and the layer blend mode set to Color (that will minimize side-effects in tonality and somewhat de-emphasize the spotty stains). That, unfortunately, loses the photo tone altogether, so you'll need a second Hue/Saturation layer to fix that, with the "Colorize" checkbox checked. Playing around, I found that a hue value of 34 and a saturation value of about 10 gives a suitably vintage look without looking like you're trying to create a vintage look.
That still leaves you with a bit of clean-up work from the leftover staining, mostly on the dresses of the two little ones up front (and how much you do is up to you; most of the spots get lost in the fabric design) but the majority of the problems will be solved. And you may want to do some additional spotting once the colour issues are resolved; the girl at the left of the picture really need some work on her face that should be fairly easy to do if you go at it with a light touch.
I think that you may have a major problem.
You seem to be treating the photo as monochrome.
However, the yellowing you see is not from aging of a monochrome photo, but the differntially faded remains of manual "colourising" at some stage that has faded badly.
The picture below shows the colours that are present. This is not meant to be an attempt to optimise what is there but just a 'revealing' of what is present. The bows are still visibly red but it seems that the whole photo was coloured and has faded to various extents. You are going to have to decide what to do with the remaining colour.
Just monochrome converting the image will affect the relative contrasts depending on what colours were originally present and how much they have faded.
A monochrome conversion COULD look like this. Again, no effort as put into optimising this for the colour effects on contrast - it just shows what sort of result might come out of such an effort. Arguably, the photographic quality of the original has been severely impacted by the colouring process and unless the result is of great importance it is possibly acceptable to make best efforts globally. One could try masking and layering and applying different treatments based on colours and degree of fading, but it would have to be especially valuable for some reason to make this worthwhile.
For interest, can you tell us something about the photo's history.
