Colors can only be judged based on the colors around them, in relation to each other. This example doesn't give me enough info to tell you what is or isn't working. What you are doing right is thinking about warm and cool colors, which is good. Know that color has three other attributes: hue, chroma, and the most importantly, value.

If you're new to colors, I suggest find a colored reference and try to copy what you see first. If you can't copy what you see, you can't possibly invent.

Hey desmond68, Overall this is pretty nice, and has nice line quality. There are some issues like tangents, parallel lines and stiffness. I did a quick sketch to show you some of these issues, I hope it helps :)

Hey Desmond, I really like the gesture you put into this drawing, nice job. As far as your rendering goes, what I notice the most is that the values are all over the place. The best way to keep that from happening, is to focus just on the shadow, no halftones. The way we determine what is shadow, is squint. What ever looks dark when squinting, is shadow. Then, you can look for halftones that help give dimension to the shapes. I did a quick sketch to show you what I mean, I hope it helps :)

Hey desmond68, Nice (creepy)(I hope it isn't a relative) portrait. You controlled your values pretty well, but as mentioned, there are areas that could use some darker values and contrast. What I did notice is that you lost some of the structure of the body. also, by drawing a center line, it would help you make sure you are lining things up on the jacket properly. You also want to make sure you pay close attention to the distances between the different features of the face. You have his face narrowing at the bottom, but it is actually pretty square. Also the distance between the nose and the mouth and the chin, and the width of the mouth. I did a quick diagram to show you what I mean, I hope it helps :)

This is really some great reference, It shows off the back muscle really well. With this drawing you seem to be having some proportion issues, and some unfamiliarity with anatomy. I did a quick sketch to show you some of the things that I think you could work on going forward. Don't forget to really push the gesture and make the underlying structure solid so that you get a feeling of 3 dimensions in the forms. I hope this helps :)

The only suggestion I have is darkening the left side of his coat. When I squint his hair looks a shade darker than the coat and in your reference they are the same value. Unfortunately I'm not well versed in clothing yet but the folds look accurate. I would say to soften the contrast maybe. As a composition those folds are asking for my attention when I think the attention needs to be the face since it's a portrait. That being said I think this portrait looks great! Your values in the face are great and I can read the form clearly.