We've
touched on this before, but basically it's a bit of a nonsense to say that Hergé worked in any specific style.
Joost Swart coined the term "ligne claire" in 1977, after the last completed Tintin book had been published, and while his definition covers some of the aspects of Hergé's style, it also ignores or leaves out a lot of what Hergé did over the course of a fairly long career, and none of Hergé's work really adheres to Swart's
post facto "rules".
For example, Hergé did use shading and hatching - from cross hatching on the suits of Haddock and the Detectives in many of the early books (to define where the characters' arms were), to subtly painted shadows beneath characters in
Crab, to the night walk through the forest to the gypsy encampment in
Emerald, where the shadows are inked in. He continued to hatch dark shadows beneath cars for the whole of his career.
He didn't use a constant line weight, as Swart says clear line should, but varied it considerably, depending on the effect he was looking for - thus Snowy is almost always given a lighter stroke than Tintin; this was to give the little dog a fuzzy, furry look, and is also found on the bear cubs in
Destination Moon.
He also used colour modelling as you describe - not just on Tintin's cheeks, but for his hair (which is only shown in the colour scheme, not in the line art), and often had an air-brush effect to vary flat colour: the under-water sequence in
Red Rackham's Treasure is probably the most extensive, but you'll find it in the way the parabolic reflectors are made to look metallic in
Calculus Affair, and as late as
Picaros, where the abstract Henry Moore-inspired statue is given incomplete black lines, and is in fact mostly given definition by gradation of colour.
So the short answer is that while the clear line defintion as proposed by Swart may be an idealised scheme which reflects much of Hergé's technique, and a simplified expression encapsulating some generalized observations of what he did, it is in fact not a proposition of the
actual method Hergé used, nor is it right or useful to suggest that Hergé would have wanted to be constrained by having to abide by it.