Steps to reproduce:

• Spawn in a mob (I used a pig)
• Name it by any means
• Type: /scoreboard teams add Red
• Type: /scoreboard teams option Red color red
• Type: /scoreboard teams join Red @e[type=Pig]
  The Pig's nametag will NOT be coloured despite it being on a team which has an assigned color.

While this is a bug imagine how much easier it will make it to make coloured name tags for entities! If you want this then please do vote for this bug!