Actually, it's a different "type" of tattooing. Military Veterans who become cops usually have a unit tattoo on the forearm or shoulder. Sometimes they'll have a tat on each arm. But seldom do you see them with "sleeve" tattoos that cover the majority of the arm down to the wrist.
I am talking about forearm tattoos.
With that said, Officer Keith Roach is a fairly dark skinned African-American. Tats aren't going to show up on him very well. That means that Roach must have gotten a very good look at him to see the tats.
I can see the prosecutor challenging the jury to consider that if Thomas could see the tats well enough to decide Roach was a criminal, then why could he not see the uniform good enough to decide Roach was a cop?