Hi everyone, this is a colored version of a sketch a did years ago of my grandpa. Please give me feedback on how to make it better.

Cartoon version of my grandpa
It looks like the biggest turn will be some simple anatomy changes (while keeping the integrity of the caricature type of style).
The first thing I would do is flip it horizontal. You’ll notice parts that have one side higher than the other. The next thing I would do is look at reference pictures of hands and feet (or shoes) and use them toward restructuring the hands and feet. (the hands and face are typically what someone notices first when it comes to anatomy, so those are very important to get right!)
Don’t finish those changes, instead I would keep it sketchy and post an update. When all of your rough parts look right, then you can continue to outline/shade which we can help you out with as well 
I agree with the previous comment about anatomy. Try making the pose yourself or if possible ask your grandpa to pose. Sketch the pose out several different times before you decide to refine it into a finished drawing, it will keep the energy of the pose this way.
As far as your painting is concerned, watch where your light source is coming from, you have a lot of unnecessary highlights. When you pick a base color for the skin or the hair, try to shade in a slightly different hue. For instance, make the shadows slightly more blue and make the highlights slightly warmer. That way it won’t look like brighter and darker versions of the same color and look more like real life where simple looking things have a vast range of color. Does this make sense?
I can see your grandpa is a real character and I love the way his eyes bulge through his glasses. Good luck! Can’t wait to see the finished product.
Other than the comments you already got, you should think about the surface properties of different materials. Right now, you’re rendering all the surfaces the same way, regardless if it’s cotton, leather, skin, metal, etc. Different materials respond differently to light, with different quality of highlights due to different specularity levels and textures.
Also, stop using Dodge/Burn to rendering lighting information, because it is not the right tool. It alters both the values as well as the saturation, and that is not what you should be doing. Actually paint the different values.
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