When you're doing a study, it's very useful to have your copy and the original side by side, and frequently step back and squint to take measure of how accurate your values, shadow shapes, and drawing are. It will really help you gauge where the inaccuracies are. And the most important thing is getting that drawing right, those shapes in the right place before you move on to the colors.

when it comes to painting, the first thing you wanna consider is the light source. what color is it? is it the sun or studio light? is it hard or soft? this is mandatory in every realistic portrait paintings ever. in the original, Cesar Santos used a semi soft blue-ish ( or maybe white ) lightsource ( based on cool highlights on the forehead and the soft shadow ( the ambient lights is also quite cool ). your painting looks quite yellow overall, especially in the highlights. the overall pattern i'm seeing in the original painting is "cool light, warm shadow". his skin is also kinda dull in your painting, thats because you use very muddy gray for the shadow. but in cesar's artwork, he saturated the shadows, the color he used is a dark saturated orange. if you have photoshop or something, you can color pick both your painting and Cesar Santo's artwork to see the differences. if youre wondering how Cesar Santos's painting is so vibrant, that's because of "color notes". the colors are always shifting in hues, saturation slightly ( while the values stay mostly the same ). most prominent in the beard. you can see bright yellow, gray red, dark orange, dark red, dark gray purple, bright gray purple, even gray blue. but from far away, the beard seems gray. you can add a lot more variations in your painting, making it more interesting. ( if you dont what that is, here's Marco Bucci's video explaining it: https://www.youtube.com/watch?v=jFoKmX0LfCs ) the hairline should be softer. the proportions are slightly off, you can fix this by measuring, or if you have digital art software. put the original painting on top of yours and flip between them to find out your mistakes. notes: i mentioned about the "cool light, warm shadow" but you don't have to conform to it. you can see that some parts of the old man's beard ( in Cesar's painting ), he used warm orange, and for the brow bone highlights, he used bright orange. but the painting we can still tell that he used white / blue light source because Cesar used other factors, like how majority of the colors are in red in hue ( not in yellow like your painting ), other highlights are bluer, especially that forehead and the nose, and blue color notes. sorry if this critique is super confusing, i still have a lot to learn but i hope this critique was somewhat useful