For the sake of brevity(-ishness), I will concentrate on a single image.

The first image in that batch has 474 colors (according to http://yanrishatum.ru/pj/ - the PJ Image Specs tool ). It needs…decidedly less than 474 colors. Given the number of colors that sit above 1% of usage, you can reduce the color count to at least 24 (and possibly lower than that).

Pixel art is a form of minimalist art. It is about finding a way to express your idea with as few colors as necessary. A given image may have need for multiple ramps of a specific color. Perhaps you need to display different kinds of materials. Otherwise, a single ramp will work fine — if you even need a color ramp at all. The key is in how much detail you truly need for your image.

Look at the list of colors in that classroom image. I would like you to go through each minor color you used for one spot or one small detail. Ask yourself this: “Can I use a color I have already used in this image for that detail?” You will most likely find that yes, you can. (For example: Do you need a half-dozen shades of red for each individual red item when a single shade could do the same work?) See if you can re-use your most-used colors as contrasting shades/highlights. For that matter, see if you can collapse colors that are near-identical to each other into one color. (For example: #8a5e2f and #8b5e2f are practically the same color, so try using one color to replace the other.) If you absolutely need a new color, make sure you are adding it only out of necessity.

The overall design of the classroom image is good. It’s readable and generally well-colored to the naked eye. But here on PixelJoint, the standards are a bit more…technical than “does it look good to the naked eye”. It’s one thing to have a good design. It’s a whole other ballgame to put it together without using nearly 500 colors where a couple dozen colors or so could do the same job. Palette control is about knowing which couple dozen colors you truly need to express your idea.

Telling is one thing. Showing is another.

That is part of your original classroom image. It uses 68 individual colors.

That is a quick edit I did of that portion of your original image. I took out a few details to help reduce the color count and merged colors that were practically identical. The edit uses 13 colors — more than five times fewer colors.

Ignore the changes in details and look at the broader overall image. Can you tell that the primary shade of brown on the desks is one color (#8a5e2f) instead of two (#8a5e2f and #8b5e2f)? Can you tell that the entirety of the visible floor no longer uses 14 colors, but 5 — one of which is a recycled darker shade of brown?

If you want to have those details I removed, ask yourself if you need new colors to express them. Can you use an existing color to add that detail? Also ask yourself if you need the detail at all. Simplicity can help with readability. And above all else, remember this: You don’t need to use several dozen colors if one dozen can do the same job.