Actually, the products are available. They are just dye the right hue (colour) and thinned with the appropriate thinner, e.g., water, alcohol, white spirit, naphtha, etc. You may find you can use a single dye, e.g., a walnut dye, or it may be that you have to mix two or three dyes together to get the right colour, i.e., to match the colour of the heartwood, which you may have already altered with dye or stain. Usually if you dye or stain the heartwood you colour up the sapwood at the same time because you colour up all the wood to start with.
It's then that you have to use the sapstaining dye to get the sapwood to match the heartwood. This generally means localised application of your dye, i.e., getting it on to the sapwood, but not on to the heartwood, often using a brush and a cloth to wipe off as needed, although it can be sprayed on too. Even here you'll find you need to adjust the dye strength according to the paleness of the sapwood compared to the heartwood. The sapwood tends to get darker as you get closer to the heartwood because usually sapwood gradually changes to heartwood over several years.
Success generally comes from carefully feathering in the sapstain so there isn't an obvious change in colour and gradually building up the depth of colour. There's also a technique of toning which is where you add dye to the finish and apply this locally, usually with a spray gun, to adjust colour, and this can be useful to get things to match better.
I've had reasonable success doing the job, but there are finishers out there much better at it than me. One of the longer term problems tends to be the way the wood reacts to UV rays and oxidation over the years. What can happen is that the heartwood will probably change colour over time, but the sapwood colour doesn't change the same. So what was a really good match at the beginning can become a less than good match a few years later. Slainte.
