I can dig it. Layers are totally useful. Changes happen all the time. Especially when on the job and layering each element will save you tons of time without destroying stuff. What you refer to as 'alot of work' is merely a form of organization. When you practice any method it becomes natural. Honestly this will save your ass over and over if you are being directed by a client or boss.
I can agree that in some forms 1 layer is neccessary or prefered. But most of the artists I know whine ALOT when they have a 1 layer painting and are asked to change this and that. Cause its hella work and hella messy to change. I often find that artists don't want to use layering methods because they have really bad hotkey setups and they find it annoying to click on stuff. Try setting it up like a 1st-person shooter.Use the WASD QE ZC X RFV keys and then all the combinations of ctrl alt and shift with those keys. keep what you use most on the left side of the keyboard and what you use least on the right side. Example: in photoshop 'ctrl-Q' by default is Quit. This is completely useless on the left side of the keyboard (and at all as a hotkey). I put Make New Layer on ctrl-Q. It is very easy to push and without even thinking about it I type in a name (sometimes just 1,2,3,4,5) and hit enter and continue drawing. Theres a ton of other hotkeys that can be removed and replaced with much more useful actions. I suggest everyone read thru the list of every program you use and make a very nice compact custom set.
But no offense to anyone. Smiles all around.
In honor of good old fashioned education OriginalAdric should try Evileye's process. And Evileye should try OriginalAdric's process. If you don't see why something is useful at least give it a couple of gos.
So I played around with your art a little.
I typically use Overlay layer mode to do shadows and lighting. It mixes colors well with others without making them too muddy and white and black up and down the luminance in a (usually) pleasing way. When ever I feel like my photoshop art is too mid range I throw on a couple of Overlays and it gives me a lot more key to work with.
Your drawing could probably stand 1 or 2 more levels of highlight. And maybe a little bit stronger bounce light. The final mix almost completely erases it. I drew in a little of both but you could do it cleaner and more precise to your liking. Multi color shadows never hurt anyone, but if its not your thing its cool. You can base the multi coloring off of reflected light, opposite light and shadow colors, or just colors that 'do something' together. Depending on what I feel like I use 2-4 colors in a shadow. Comparison is always the best way to go. just shove all the layers into a group, clone it to the right and then change the colors and see what happens. Clipping masks and pixel lock are the best ways to do this like you stated in your post.
I also really like the original drawing so I was curious to see if that would add anything to the composite. The bright version uses the sketch with Overlay and the dark version with Multiply. I also used Adjust-Hue/Saturation and checked 'Colorize' to play with the sketches line color. The black pixel lines might like some color as well even if it is very close to 0,0,0. I didn't try that in my edit. oops.
Your art is educated. You can do much better than my quick edit. Just try some stuff and go for it.