Function PlayerTextDrawBackgroundColor adjust the background color of a player-textdraw.
Function PlayerTextDrawBackgroundColor was added in 0.3e and will not work in earlier versions!
If PlayerTextDrawSetOutline is used with size > 0, the outline color will match the color used in PlayerTextDrawBackgroundColor. Changing the value of color seems to alter the color used in PlayerTextDrawColor.

(playerid, PlayerText:text, color)
int playerid The ID of the player whose player-textdraw to set the background color of.
int text The ID of the player-textdraw to set the background color of.
int color The color that the textdraw should be set to.

Return Values:
This function does not return any specific values.

new PlayerText:MyTextdraw;
public OnPlayerConnect(playerid)
    MyTextdraw = CreatePlayerTextDraw(playerid, 320.0, 425.0, "This is an example textdraw");
    PlayerTextDrawUseBox(playerid, MyTextdraw, 1);
    PlayerTextDrawBackgroundColor(playerid, MyTextdraw, 0xFFFFFFFF); // Set the background color of MyTextdraw to white
    return 1;

Related Functions
The following functions may be useful, as they are related to this function in one way or another.