After seeing a lot of the responses, and considering this goes along with a lot of my own work which I currently do in sharing free techniques, tutorial videos, actions, photoshop files and more... I've decided to write an action which does something similar to using the Gradient Map approach to this, but with precision and accuracy (as the Gradient Map in Photoshop compresses the greyscale range even with a Black and White point set at the ends - it changes the original, which is another topic about issues with Photoshop's tools.) I prefer to re-colorize gradients instead of using masking, because transparency of masks leaves parts that do not get adjusted between the original and the mask, because of transparency being used. A lot of my techniques go into extracting the luminosity or chromaticity masks as layers then using blend modes and color fills to re-color the parts however you want.
If you don't really need precision, then gradient map is the easiest of all the solutions, as long as you know how to use it and set it up.
I've attached an action I just wrote, which uses a few techniques... First you just use your foreground color picker, or the eyedropper, and sample the grey-point of the image you want to be the colorizing point. Then run the action - there is a Black and White one, and a Black, Grey, White action - similar to a Black and White gradient map, or a Black, Grey, White gradient map. The action duplicates your original document so as not to destroy any unsaved work, then takes the image and converts to smart-object which you can change later, and takes the foreground color set to the grey point you sampled and makes that a smart-object as well - so you can change it later also - then goes through the steps making a Black and White colorizer and a Black,Grey,White colorizer. Once it is done, you can change the Color Fill "Colorizer" layers to any colors you want, and everything will blend accurately from one to the other like a gradient map but without changing the levels of the original.
Attached are the action, a greyscale test file, and the smart-object file which can be used to just replace the image contents and also change the custom grey-point level. For example after the action runs also, you can go into the grey point smart-object and change it to a different level, but it is a pixel filled layer, so just set a new color fill layer to the desired grey point which is easier to click and change, then simply save that and it will update the other layers to the new grey-point.
It is more complex in how it arrives to the result than using Gradient Map, but it is more accurate and precise in the blend, plus it requires no knowledge of how to use the gradient map, all you have to do is set the foreground color to the grey-point you want to be colorized... then change the colorizer layers to whatever colors you want. I have it set by default to be a few different colors so you can see it remapped to new colors... otherwise you can set the Black colorizer to Black, White to white, and the Grey colorizer layer to the original grey level you selected and the document will look exactly the same as it started. If you want masks I have a free luminosity mask generator on adobe exchange and my own site already, but I was planning on making something like this anyway for a custom-grey-point colorizer. It uses some tricks like adjustment layers, blending modes like divide and difference, and knowing how to re-blend colorized layers together correctly with groups. I will edit this post later to also include a tutorial video I'll make about how to use the action and smart object, as well as how to actually write the action itself, along with showing some of the other techniques like gradient map, using a single fill layer set to "overlay" blend mode if you only want to change the grey-point to a color and not the black or white, control-selecting the RGB channels to get luminosity as a selection, calculations etc.
A simple question can lead to multiple ways to arrive at an answer but opens the door for plenty of exploration and learning and having fun being creative in different ways. Hope this helps in your quest, use the action and file and techniques however you want, much of my work is open-source to be used freely without attribution.
I can't attach the actions file directly to this reply, but I'm including links to the test file, smart object file, and action file on my creative cloud for downloading to yours or directly, anyone can download or use these if they like. I will come back after making the video and post that also.
Greyscale Test Image
Greyscale Smart-Colorizer Smart-Object Photoshop File
Grey Point Smart-Colorizer Action File
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