Synonyms only work for product names. If you need to create a synonym for a brand or color, a work around is required.

Step 1: Create a new derived property and select the type Replace. Select the property for which you need to create a synonym.
Make sure you choose Skip value for the original values you don’t want to replace. Click Save.

Step 2: Replace the value with the synonym.

Step 3: Add the derived property to the search properties. The synonym will now generate the same search results as the original value.