First, you need to have some tags on the concepts (keywords/topics/bubbles) in your map.

👉 See How to use tags

Then you can change tag colors as follows

1. Left-click on the concept/relationship whose color you would like to change.
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2. The tag list will appear on the left in the Concept notes tab of the sidebar. (pic. 1)

3. Click on the coloured circle next to the first tag.

4. Select the color from the Quick selection or choose one of your own (custom).

If the concept contains more color tags, only one of them is marked as a main tag which can change the color of the concept. To change the main tag, select a tag and open its color options. Then click on the option "Main tag for this concept".

Pic. 1