As promised, here's an attempt at a step-by-step guide to the evolved painting method using Contrast Paint and Army Painter washes followed by acrylic touch-ups / highlights.
As I was painting, I did think of a new name - the "Henry Cooper" method - as in the Brut adverts "splash it all over" although to be fair the later bits are more Tommy Cooper - "just like that" 😆😆
Minifigs 15mm Celts (Gauls / Britons) - grey spray primer.
Step one Contrast Paint
I just try to keep it as neat as possible, but inevitably some colours splash over parts you don't want. Instead of touching up - re-priming and reapplying Contrast, I just cover up the errors with step 2...
Step 2 Army Painter ink wash - in this case Dark Tone t replicate the black undercoat method. You could use Strong Tone, especially for flesh. Note it results in a "what have I done...." reaction first time. If possible, I try for a "pin wash" in the deeper recesses, but often, as here, the whole figureis covered.
Step 3 highlights in acrylic - here I have just done flesh simply to show how just this lifts the figure.
Now the same for the main colours...
Step 4 Fine detail - in this case the black and white, with shield designs in black, followed by white, red and yellow normally - it also allows for a mix of white, yellow and red foe hair and moustache highlights
Final stage, metals, a lighter flesh highlight to faces and raised areas and any corrections. Also the base got a coat of Vallejo khaki - read somewhere covering the paint splashes on the base makes a big difference.
Hopefully this is of interest - just my way of trying to match black undercoat figures.
The Contrast Pain and AP wash were a couple of sessions after work, maybe an hour at most. Rest of painting was about 4 hours yesterday and two today so maybe 7 hours total for 20 figures - roughly 21 minutes per figure - more than the WW2 in simpler uniforms.