Here are some Colour Recipes for Horus Heresy Mechanicum from Gallery: Forge World Zhao Arkhad. Please take note that this is a simple colour scheme, not covering multiple overlapping layers and blends in between, that lead to the final product. It is supposed to be used as guideline not a step-by-step.
BRONZE armor:
Black Undercoat,
Mix Rust (val a) 1:1 Metallic Black (val a), *
Bright Bronze (val a), * zenithal
Shining Silver (ap), flbr
Strong Tone Ink (ap), wash / *
Shining Silver (ap), l&p
Soft Tone Ink (ap), wash dedicated
… GREEN metallic:
Jade Green (val), bl
TURQUOISE clothes:
Coal Black (p3),
Strong Tone Ink (ap), wash
Coal Black (p3),
Mix Coal Black (p3) 1:1 Turquoise (val), flbr
Mix Coal Black (p3) 1:2 Turquoise (val), bl
Turquoise (val), l&p
Ice Yellow (val), bottom drbr
SKIN:
Slaanesh Grey (gw),
Flesh (val a), flbr
Pale Flesh (val), l&p
Strong Tone Ink (ap), wash
Pale Flesh (val), l&p
BONES:
Flayed One Flesh (gw),
Ivory (val), flbr
Strong Tone Ink (ap), wash
Skeleton Bone (ap), l&p
Off White (val), l&p
GREEN osl:
Duck Egg Green (val a), *
Light Livery Green (val a), *
Off White (val), l*p
Waywatcher Green (gw), bl
DESERT bases:
Desert Sand (val texture), texture
Khaki (val a), * stones
Ice Yellow (val), drbr
Tufts:
Wasteland Tuft (ap),
Steppe Tuft 12mm (Paint Forge),
Steppe Tuft 6mm (Paint Forge),
l&p – lines and points,
p – points,
dl – deep lining,
bl – blend,
gl – glaze,
drbr – drybrush,
fltbr – flatbrush,
lobr – loaded brush,
stpl – stippling,
*Airbrushed (with multiple layers and mixes)