My interpretation of events is that is an often accepted fact that black in its basest forms in any artistic venture draws any colour and light from an object.
Consequently many art schools refuse to let at least early students use black pigment in any way until they get to grips with the reality of the colour.
Grey is always mixed from other colours (perfectly easy really) and consequently you end up with colours that are a greeny grey or a bluey grey or a browny grey or whatever, these colours are easier to get good shading and highlighting for and give more depth and life to a subject - even if you are painting a concrete block life and depth is still important believe it or not as cold, severe and mechanical shades frequently just look odd.
I suspect that Shadow Grey et al were mixed to be a bluey grey because looking at - for instance marines just painted in pre heresy Spacewolves armour of battleship grey - they look like grey lumps of plastic and not miniature space soldiers.
Also I suspect the same sort of thing was true for Darkangels too, they used to be black but just black is really really dull (almost definitively so some might say) so a black-green allows for more interesting highlighting and depth of colour glowing through rather than just an absence of pigment sucking in the light and looking like a chunk of coal.
As to why they thought to present these colours as they are rather than just giving us codex grey and letting us add a dob of colour to it ourselfs I don't know. Maybe they are trying to better us in some artistic way? Maybe they realised that Codex Grey is a nothing colour and its prolonged use just kills any paint scheme stone dead? Maybe they just enjoy seeing people wriggle when they cant just pull a gobbet of paint straight from a pot and have to use a pallette instead.
I wouldn't disagree with you, its all just personal taste after all. I used to do Darkangels years ago so I would have had to repaint all mine form black to black-green if I hadn't just ditched all all of them instead.
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