Only colours which appear in the pallet.

There are more colors than what you see initially... if you click on "Solid" it will open a dropdown menu with more color palettes. YMMV on mobile though. You can also get more colors by checking the Legacy option. Finally if the bricks can be custom decorated, you can upload a texture image with a solid color. That should paint the entire part.

Here's a model with the normal light and dark bluish grey, but with a custom 50% grey in the middle. Of course, this middle brick is stuck this color now. In order to change the color of it you would need to upload a new image... but it does get you any color you need.

Here's the link:
[url]https://www.mecabricks.com/en/models/Dp2l9mgQjMV[/url]

[mecabricks]Dp2l9mgQjMV[/mecabricks]

Thank you @B1TRash and @XForce198!
But how would i add an image?

How to Make a Custom Decoration