When you have absolute confidence in your ability to correctly copy proportion and value* for any number of lighting setups using the comparative measurement method.

To be frank, I think it's a good idea to fuck around with it in traditional media a little all along the way, just for familiarity's sake. However, this should all be kept separate from your real study/practice, just more for 'fun'.

Whenever you're happy with your forms and values. There isn't a defined spot.
Color is tricky but once you've got a good idea of value it's a lot more intuitive. Do some studies of stuff with good colors to improve your eye for color (namely: recognizing when something you assume is red is actually brown, purple, etc but just looks red in context).

>>2356824
oh and on value, value is a little tricky, as there really isn't any media, other than maaaybe digital on an amoled screen, that can truly replicate the full range of value you can see. So you kinda have to make decisions on what range of value you're going to choose to represent accurately. You'll have to sacrifice either the upper end or lower end usually.

>>2356812
Things like this go hand in hand, don't wait for you to start implementing colors into your everyday work because, even if indirectly, one thing affects the other.
My brother's been a professional musician for around 7 years now, hasn't composed his first piece because he doesn't feel ready and wants it to be perfect, that imo is one of his biggest mistakes if he ever wants to get his work out there.

if you never do it you'll never be ready

being able to copy what's in front of you with some resemblance helps, too