This command was draw a line in 2D space. Specifying the colour parameter will overriding the colour set with D3D_COLOR.
Specifying two colours will generate a line with a gradient from the first colour to the second.
D3D_LINE X1, Y1, X2, Y2
D3D_LINE X1, Y1, X2, Y2, Colour
D3D_LINE X1, Y1, X2, Y2, Colour1, Colour2