edit, keeping it within your style and mood, not the only approach but hopefully helps you along. contrasting each garment to clearly seperate his elements
some sound suggestions from Mr. Beast but he looks bored/like he's falling asleep in that edit, and the handle of the foreground blade is hard to read, I can only assume it's a cool metallic reflection but it doesn't read as such, I believe the monochrome nature of the clothing isn't the most compelling choice either, and the angle of the background blade looks bizarre in relation to the handle, the shadow tone of the pants doesn't ramp very convincingly either, relatively nice colour choices otherwise, some suggestions and further editing (and having some fun on the end, with rather rushed colour choice)
^-speaking of 'luminosity' (these greyscales are raw (mathematically calculated) luminosity which isn't strictly as accurate as perceptual), there are a few ways to calculate and gauge the brightness,
one method if you are using a multi layer program is to create a layer of solid black or anything with 0 saturation, and using the layer influence mode saturation, flicking it on and off as you work with the colours to judge your colour choice is the quickest way to gauge,I've only really done it with digital paintings though but raw values are for the most part really useful and you can easily identify when it's not perceptually correct. Greyscaling the image depending on the program will sometimes try to use a lighting model of some description to get averages for the greyscale value, and most programs have desaturate parameters like luminosity or simple value, and also an average of the parameters which is often pretty accurate perceptually.
as for more guidance you should try describe or atleast stop and think for yourself what style and mood you are trying to capture with your colours, and if elaborated we could offer much more directed critique. the colour scheme itself brightness values aside isn't too bad by itself but looks rather like unmixed paints, and you don't necessarily need to make it darker like in the edits either, is this by itself? or are you planning to integrate it into a composition? if you are then a rough colour choice not more complicated than what you've already done is ideal and then when you have your scene you can figure out the lighting and mood and ambient colours etc