I try to keep it as clear as i can to help you out.
People that say painting from colour is better mean it only because it is hard to convert grayscale into colour. I personally find it very distracting and overwhelming trying to tackle ALL the things like light and shadow, anatomy and perspective, composition AND COLOUR at once.
So for training your eye and learning fundamental principals of painting I'd recommend you to do more greyscales until you will get a good grasp of it.
(of course you can combine greyscale practice and then do the same piece in colour and compare it, or you can keep doing colour.. there are tons of options and you are in your own hands the question will be more about how efficient it is for you )
Anyway. going back to your drawing here.
I haven't pushed this paint-over as dark as Marc's version is but hopefully I've captured the idea of how to make it looks better.
Main key is Your light source. Always consider it and always think of what will face the light and what not. By doing this you will achieve more 3d look in your paintings.
To do the blocking out part few moment that you need to consider are anatomy and your light source. How each plane of the face will reflect the light.
And the last main thing is that Your value range never will be as white as the light source unless it has a 100% reflect-ability, even eyeballs will be darker.
Preserve all the light reflexes for last and go darker gradually as you work. Don't go 100% black in the beginning of your work.
The face i did doens't look 100% as Marc's piece but again. I hope i captured the main idea.
And I gonna link you few videos in case you will find it helpful.
How to block in
More on blocking in and edge-work
She has even more videos on her channel explaining all the nuances as she critiques other ppls work.
How to colour skin - starts at 5:47
Marci Bucci also has a video on Light and Shadow from his series of 10 minutes to better painting.
Happy painting and I hope that it was helpful