Function PlayerTextDrawBoxColor sets the color of a textdraw's box.
Function PlayerTextDrawBoxColor was added in 0.3e and will not work in earlier versions!

(playerid, PlayerText:text, color)
int playerid The ID of the player whose textdraw to set the box color of.
int text The ID of the player textdraw to set the box color of.
int color The color to set. Alpha (transparency) is supported.

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

new PlayerText:pTextdraw[MAX_PLAYERS];
public OnPlayerConnect(playerid)
    pTextdraw[playerid] = CreatePlayerTextDraw(playerid, x, y, "...");
    PlayerTextDrawUseBox(playerid, pTextdraw[playerid], 1);
    PlayerTextDrawBoxColor(playerid, pTextdraw[playerid], 0xFF0000FF); // Red box with no transparency
    return 1;

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