There is no slap-chop, there is only Grisaille!
With the advent of Contrast and other 'speed' paints, a 'new' painting style has emerged - the 'Slap Chop' method, using a model which has been 'greyscaled', from dark grey or black with several highlights of grey up to a final white highlight. As with most innovations, it is based on something far, far older.
The technique is simple; colour washes or inks are painted over a surface which has already been grey-scaled. The grey colour's light edges and darker recesses help to bring depth and variation to the wash, as the colour goes over the surface. The technique is very effective, giving dark, mid-tone and highlight in one stroke of the brush.
This method is called Grisaille (Pronounced ɡrɪˈzaɪ) and dates back to the 14th century AD. While it was used to create sculpture-like black & white paintings, it was also (and primarily) used to draw up a painting before thin glazes of colour were applied.
I recently used this technique in my Advent Calendar posts in the run-up to Christmas. There, I took some plastic Warlord Bersaglieri (with a few parts/conversions from the Atlantic Italians) and painted up a squad and a half.
I started the uniforms using Army Painter (AP) Pallid Bone. The occasional figure was given green-grey trousers (some European theatre kit). For this AP Camo Cloak was used. The Puttees used Vallejo Green Grey (VMC 886).
The boots were painted in AP Hardened Leather, which was also used for the belt and pouches. For the helmet, I used Citadel Contrast (CC) Skeleton Horde, which nicely offset the helmet and uniform colours. The feathers were CC Black Templar (still my favourite black), and the flesh colours were either AP Crusader skin or CC Gulliman Flesh.
The models were finished off with some wood and metal effect on the carbines. The bases were finished off with AK Desert Sand basing, plus a few rocks and scrub plants. Overall, they look good on the tabletop and were fairly quick to do.