If you use a decorated brick in your build, for example 3001d1 and export your model to collada you can then "repaint" the decaration .png file called 3001d1.png that you find in maps decoration folder. I use Inkscape and Gimp for all my decorations. Just make sure to not change the size of the png.

You will not be able to use your own decoration in mecabricks but you can use it in blender by just replaceing the original 3001d1.png with your own decoration with the same name(3001d1.png)

Thank you for this idea.
Unfortunately, I'd have to use the color-limited decorated bricks - which is a problem because I can't change their color without much time.
I'd have to repaint every single brick, I can't do that. Is there another way?

I'm not a Blender guru, I just follow Scrubs and the others guides and tutorials, so i don't know another way.

When you do custom decorations for use in Blender (or other 3D software), you can use any colours you want. You are not constrained by anything. The background of your image will drive the base colour of your brick.
There aren't any magic tricks for this. you'll have to do that manually.

Hi again,
even if I use the suggested method - I'd have to create about 30 different decorations for a 2x4 brick. Due to the limited number of decorated bricks, I can't use them all at once since replacing the decoration will result in a global change, won't it?
I started with creating 3D-Texts in Blender and adding them to the bricks. It costs much time since I don't know Blender well enough - but it works.
Thanks so far, maybe I understood the whole decorations wrong!?

What you can do is to just duplicate the same decorated brick in mecabricks and import the model in Blender using my add-on. All bricks will have the same unique decoration coming from Mecabricks.
From here, for each brick, duplicate the material by clicking on the '+' button (Add a new material) next to material name. In the node section, replace the macabricks material file by yours - remember to change the base colour hex value by the one of your decoration.