Hi there again! Long time no see! Good to see you got back to pixel art!
I'm going to be a little harsh here, so bear with me -- I just feel honest critique goes a long way when trying to improve on something.
So, first thing, is that you've done pretty well with 12 colors to get across what you were trying to convey. Unfortunately, I feel like your palette could have been used a little more effectively in the sense that you seemed to put most of it toward shadow gradients, etc. These entries actually caused you to have a 13th color somehow in the darkest shadows on the floor. I suggest you get a program like Graphics Gale if you haven't already (it has a feature to count colors in the image). I actually removed a color in my edit that could have been rendered with an existing color (i.e. the wires' gray color), so my color count is now 11 after removing that extra color in the shadows.
You went pretty heavy on the dithering and it mucked up the clarity of your image, so I tried to show you how it could look better with less dithering and more solid colors. There was some dithering I didn't even see zoomed-in, which means it really was unnecessary. I do appreciate the subtle color transitions you were aiming for with the dithering, but with color-restricted pixel art, this is not usually something you want to devote specific colors to, but instead you want to make sure all the colors you have evolve naturally into one another, creating a dynamic, but organic, palette -- and you've accomplished this pretty well, so congrats.
Not sure if you were aiming for a great composition for this, or were just practicing forms, but wires were definitely useful in that. I got the feeling of a Resident-Evil sort of house-laboratory feel from the colors and dithering, so I edited accordingly:
The biggest trouble I had with your image was the subtle transitions that were very close to the same color except normally in hue. Many of the colors in the dino's housing were replaced easily by other colors in your palette, which could have freed you up to use them for other stuff. I had a whole extra color leftover out of your 12 color restriction by getting rid of the gray on the wires, as mentioned before, which allowed me to use the background gray as a middle shade for the wires, since the intention was to blend them in fairly well. I stole a little focus from the main subject matter using the lighter wires, but I added more interest to the composition as a whole in doing so. It also gave me an excuse to use them to frame the empty space in the rest of the composition.
Not sure why there's a shadow being cast into the darkest color in the image, but I removed it to show a little more of the 'dark' mood you appeared to be going for with your color scheme. I left some dithering in there on the oxygen tank in case you could come up with a more purposeful use for it, since it kept the framing looking nice, but it really should be modified a bit or removed.
As you can see though, there was no need to add in more colors to use the existing elements in the scene to make it work compositionally. You can do a lot with the rendering to solve issues such as 'needing' a mid-tone on the wires. The end result was only 11 colors (I started with 15 before I removed the white, then ended up with 13, cleared a grey color and found that hidden dark grey dither in the shadows, leaving us with only 11.).
Though, as a side note, most color-restricted palettes will generally restrict to about 15-16 colors (SNES style) -- if you're going for any sort of variety in painterly detail, as you appeared to be going for, so take it as a point of pride you got this composition looking fairly workable in the 12-13 color range you did. It's possible to restrict more, and still get a pretty workable result, as NES games have shown time and time again, but for a nice image to look at, this wasn't far off -- I just gave you a few techniques you might consider reinterpreting on your own next time.