Instructions
Click "Army Painter" under the "Mulitplayer" option on the main menu.
Select "Tyranids" in the first drop down menu.
Click the plus sign beside "Hive Fleet." This adds new options to the menu along with a pop up dialog box.
Type in a name for your color scheme. Name this whatever you want.
Click one of the four colors on the menu. This lets you pick a color for your tyranid; each of the four colors represent different parts on your army.
Click "Accept" when you finish picking your colors. This creates the army and brings you back to the main menu.
Click "Multiplayer," followed by "Custom."
Click your named color scheme to use your tyranid army in the match.