Here are a couple of tips and sketches I made to add simple tones. I am still new to learning to draw with light but hope you find them helpful.
1. bring in pen or pencil doodle in photoshop
change to fit all screen maximized mode (makes picture clearer when zooming out)
2. lower opacity doodle (ex: about 62 percent) (helps so we can paint below the layer and still see sketch)
3. move the doodle above a clean white background layer.
4. start getting in the groove with a quick greying with soft brush of the forms (put this on a new layer above white background but below sketch layer).
5. further getting in drawing groove by using a couple forms of pen/pencil doodle to begin marking some lines (on a new layer above the greys and above the doodle).
6. After ex: sketching one dude or so and in pretty good groove (warmed up a bit) try lowering the pen/pencil imported drawing opacity. This helped allow imagination to take over with more inking lines.
7. go back to the greys to adjust for imagined new poses. (A tip if brush used for greying tool preset has a non 100pct opacity if tried to paint on same layer as grey would get darker spots in places instead of solid grey. So a tip is to use magic wand to sample the grey color, then with the same brush as used before change opacity to 100pct this way still can paint on same layer (normal layer mode) brush (darken mode) with solid color.
8. repeating 2-7 each time lowering opacity of imported pen or pencil (ex: until doodle opacity is 0) helps add details.
9. Maybe add a couple of whites for eyeballs. Maybe add a couple of darks for possibly clothing or maybe even add some greys to give box like appearances. (Tip for adding ex: 1 or two other shades of grey.
sample the current grey value > add the greyscale slider to photoshop colors > now can adjust the slider to right or left to get various shades of grey to paint with).