When working from ref, you should include the ref, so that people can know what comes from the ref, and what's your take/understanding of it. If the ref is one you can't post, you should mention that, so that people like me don't spend a paragraph asking for it
The middle colour on the face is rather low-contrast, and looks a bit greyish. I think a darker, more saturated (possibly hue-shifted towards red?) would improve it. You're also not using that colour anywhere outside the face.
Try to avoid using pure grey. Grey objects are rarely pure grey because light from surrounding coloured objects affects them, and pyre grey usually stands out as looking strange when surrounded by colour, like here. Adding even just a little hue can help, even if it's not "true to life."
The crop you've chosen is one that makes shading the body (=shirts) tough, since we can only see the flat part of the torso, there aren't any interesting form shadows, only cast shadows from the head, collar, etc. To make the shirt look less flat, you could add a subtle pattern to it (pinstripes, plaid, etc) and show that curving around the chest and shoulders.
Here are some ideas regarding the colours, hair, beard, and defining the eyes, ear, and nose better:
Since I don't have the ref, they might not look like themselves anymore, but I hope you'll get some use out of it anyway.