Your colours were kinda monotone (at least more particularly in v2 which I had saved to work on
); as long as you keep the local colour (as uninfluenced by lighting) around the middle of the ramp you can retain the impression of it being red…without everything being red. I desaturated my lows ‘cos less light = less colour and moved towards yellow in the highs ‘cos that’s a nice fiery look, and so they don’t have to be ultra-saturated in order to contrast (yellow being inherently lighter than red) while adding white would lose colour identity and look drab.
There are a few things I’d to discuss here. I’ll use the 3rd version however since it’s the latest version.

So, by local color you mean the color which isn’t touched by either light or shadow? But everything in influenced by light, so you mean the parts of the monsters which aren’t touched by highlights, right? I’m not sure I understand. Look here:
So this zone, would be considered the one with the local colors, right?
retain the impression of being red… without everything being red.
You lost me here.

I’ll try to make sense of what you’re saying.
So by the ramp you mean the variations of the local color I think. In version 3, I used this:
The arrow points the “local colour”. At that point of time, I assumed the local color was this:

But based on what you’re written, that would be incorrect as this is influenced by lighting.
I desaturated my lows ‘cos less light = less colour and moved towards yellow in the highs ‘cos that’s a nice fiery look
By your “lows” you mean the darker colors? Like these ones:
So my low-color here would be adequate because it’s fairly desaturated.
By “high” you mean “highlight”. So what you’re saying is that instead of remaining in red, you tended to shift towards yellow in the highlights. So you sort of did what I did with my highlights:
I think the issue with detail is as much the bit shapeless/awkward monster design as lack of space for relevant stuff. I dunno how the painted ref. is related to what you’re doing but it leans quite heavily on those omni-rimlights to be understood which I think would be a major pain as an animated sprite, the tentacle-things flatten/unbalances stuff in their current position (also tangent on the farmost) and the yellow wing tips distract from the glowing mouth; are they glowing too?
I agree about the rimlights which means I’m gonna have to change the concept for the monster. I don’t like those tentacle things either, they look off. I need something which won’t be too much of a hassle to animate and I can already see how those rimlights will drive me psycho when I get to the animation of the wings.
Gotta reiterate quickly what’s been said on form already; looks like you’re flood filling black linework rather than shading and I’m sure this has been mentioned before for stuff you've posted.
I do use sections which I flood fill afterwards. It might have been mentioned before but then again I’m usually very meticulous about criticism but I might have missed that. Also, sometimes, it’s just a matter of me not knowing how to do something. For instance, here I think you mean that I should share general areas first and then detail things. I’m still in the process of learning so sometimes what might be obvious for a veteran is not clear for a beginner at all (i.e. my questions above).
You could really do with practising simple volumes/contrasts without lines and sketch stuff out as rough geometrical primitives (boxes, spheres) before starting details or specifics
So what you’re saying is that I should follow more of a painting approach as opposed to a drawing one. In other words, in your opinion, I should try to create shapes and volume without using lines. I’ve read in books that painter draw general sketches before painting everything. What is the issue with drawing lines before? Do they act as crutches and prevent me from learning other fundamental things?
So I’ve decided to change the body of the beast.

So I will show you how I would normally do this.
First of all, I would draw a general outline of the thing with the 1 pixel pen in black like so:
Afterwards, I would clean up the outline like so:
(Yes, I know it’s not accurate just yet, I’m just sharing the way I work).
Keep in mind that the whole time I have my reference to look at like so:
Then normally, I would use the same tool to draw the various, obvious separations like so (only doing one for now), which I would flood fill with what I consider to be the dominant color:
Now this is what you’re saying I’m doing wrong in your opinion, Facet.

Now that I’ve explained the way I’ve done things, how would you tackle this? I’d certainly crave improving the way I create stuff, that’s for sure.
