My game involves a lot of brightly colored cubes. I want to create skins for them, however when they become a lot it gets chaotic and ugly looking with so many colors and patterns.

Here is an example without any skin:

And with:

As you can see it doesn't look that great with so many colors and objects, especially when they are even more. If I randomize it it's going to look even more chaotic. How can I fix the problem when creating the skins?

Why do you think skins are a Game Design topic? The Game Design forum is not the catch-all "junk drawer"
of these forums! Moving to Visual Arts.

Well, question 1 would be:

What is the reason your cubes are so colorful? Does the color have any meaning for the game? E.g. does it help the player read the current game state faster?

If there is no reason to the colors, why not change them to a more muted palette? Why even have differently colored cubes?

Question 2:

Why even "skin" the cubes (I guess you are talking about using textures)? Just to make them look prettier? Or does it have a a meaning for your game design?

Really, your problem seems to be kinda vague. You should try to come up with more focused questions so people here could give more to the point answers.

Different textures for different colours maybe? Brick, marble, wood... stuff like that.

Yeah, or different symbols. The one you're using is checkered, but it could also be a triangle, a disc etc. One symbol per colour, so even if there weren't any colours people could still make out the type of each cube.

Here's a quick attempt:

I'd probably reduce the contrast even more than this, but emphasize outlines:

Also, you'll likely want to pick color-blind friendly colors, especially for a game this simple.

Well, question 1 would be:

What is the reason your cubes are so colorful? Does the color have any meaning for the game? E.g. does it help the player read the current game state faster?

If there is no reason to the colors, why not change them to a more muted palette? Why even have differently colored cubes?

Question 2:

Why even "skin" the cubes (I guess you are talking about using textures)? Just to make them look prettier? Or does it have a a meaning for your game design?

Really, your problem seems to be kinda vague. You should try to come up with more focused questions so people here could give more to the point answers.

1: Yes, the cube colors are gameplay related. They have to stand out from one another so players can differentiate them

2. Both, actually. I want players to choose from a bunch of different skins they can acquire while they play

Here's a quick attempt:

I'd probably reduce the contrast even more than this, but emphasize outlines:

Also, you'll likely want to pick color-blind friendly colors, especially for a game this simple.

Thank you for the outline tip, it really makes them look better

Avoid using small rectangles, or overall repetitive patterns, within the objects. Doing so interferes with players ability to parse the ACTUAL boxes (which are rectangular and repetitive).

Use something like the curvy shapes of SotLs design. I find those greatly improve my ability to filter/focus on a specific box type, as the pattern is unique (brain doesnt confuse it with the boxes themselves)

I think giving the cubes variation in their border and/or to their textures is great for visually impaired such as color blind people, (that are way more I used to imagine)

Google neon colors and you will see that the one thing that makes bright colors look good is darkness, or to be more specific contrast.

Neon mostly uses black as contrast, however you can use colors schemes also.

A good tool for colors schemes is: https://color.adobe.com/create/color-wheel

To really make your colors stand out use a gray gradient background, some black falling leaves or other animated objects will provide a feeling of life with out conflicting with the game,