Question: color certain elements in a list

How would you highlight specific numbers in a list?
For example, using
a:=[1,3,4,5,3,4,9,4,5,3,9,2,3,4,2,5,21,32,22,12,15,3,2,5,4]

Highlight all the 5's in the list to red for example like this

            

 

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