Keyword vs data metadata in DITA?

The  element is primarily for specialization so it's probably not wise to use it directly. The  element is better.

This:

    
        
            red
            green
            blue
        
    

will render to this in the DITA-OT XHTML transform:

   name="DC.subject" content="red, green, blue"/>
   name="keywords" content="red, green, blue"/>

If you want to add tags, I'd consider using subject scheme maps, which will allow you to include a list of controlled values.

If you specialize the @base or @props attribute, you can add metadata with much more control. Here, we have a @props attribute specialized to @era.

You can then add the @era attribute to an element in a topic, or to the  element in a map.

 keys="era_attributedef">
  
  Era of production by decade and producer


  keys="producer">
     
          keys="sixties">
              keys="verity_lambert"/>
              keys="john_wiles"/>
              keys="innes_lloyd"/>
              keys="peter_bryant"/>
              keys="derrick_sherwin"/>
         

          keys="seventies">
              keys="barry_letts"/>
              keys="philip_hinchcliff"/>
              keys="graham_williams"/>
         

          keys="eighties">
              keys="john_nathan-turner"/>
         
     
 

     name="era"/>
     keyref="era_attributedef"/>