« C'est en forgeant qu'on devient forgeron. » is not colloquial at all; it's a well established saying;
(TLFi Proverbe, au fig. C'est en forgeant qu'on devient forgeron. L'habileté vient avec l'expérience. À force de forger on devient forgeron (BREMOND, Hist. sent. relig., t. 3, 1921, p. 73).
L'origine de ce proverbe semble inconnue (première trace au XVième siècle, l'internaute).
Another idea, close to the mind of this saying and also embodied in an idiomatic expression, a saying in fact, is this one;
Of the same nature, the following one can also be quoted;
- Petit à petit, l'oiseau fait son nid.
In those two latter sayings the idea is, in the words founds on the site referred to, that little by little and through infinite repetition, a small action will have an effect, and perseverance will get the better of anything.
(Peu à peu et par la répétition infinie, une chose minime produit son effet. La persévérance vient à bout de tout.)
There is stressed in this idea the underlying and understated principle inherent to the French counterpart (C'est en forgeant…) that practice will bring you to a point of perfection, a point where all the difficulties have been overcome, this being so as practice consists also in a great number of repetitions that little by little render the difficulties null, or, in other words, make your aim a reality. More precisely one should say "C'est en forgeant beaucoup/souvent que l'on devient forgeron". However, those two sayings do not stress the idea that it is the actual mingling with the particular thing you are interested in that will bring to you a knowledge of it. This is also implicit in the English saying either : it doesn't say you have to grapple with the thing itself although that is quite implicit.