Low-level graphics library.

Use this class to manipulate active transformation matrices, issue rendering commands similar to OpenGL's immediate mode and do other low-level graphics tasks. Note that in almost all cases using Graphics.DrawMesh is more efficient than using immediate mode drawing.

GL immediate drawing functions use whatever is the "current material" set up right now. The material controls how the rendering is done (blending, textures, etc.), so unless you explicitly set it to something before using GL draw functions, the material can happen to be anything. Also, if you call any other drawing commands from inside GL drawing code, they can set material to something else, so make sure it's under control as well.

GL drawing commands execute immediately. That means if you call them in Update(), they will be executed before the camera is rendered (and the camera will most likely clear the screen, making the GL drawing not visible).

The usual place to call GL drawing is most often in OnPostRender() from a script attached to a camera

static var lineMaterial : Material;
static function CreateLineMaterial() {
	if( !lineMaterial ) {
		lineMaterial = new Material( "Shader \"Lines/Colored Blended\" {" +
			"SubShader { Pass { " +
			"    Blend SrcAlpha OneMinusSrcAlpha " +
			"    ZWrite Off Cull Off Fog { Mode Off } " +
			"    BindChannels {" +
			"      Bind \"vertex\", vertex Bind \"color\", color }" +
			"} } }" );
		lineMaterial.hideFlags = HideFlags.HideAndDontSave;
		lineMaterial.shader.hideFlags = HideFlags.HideAndDontSave;
	}
}
function OnPostRender() {
	CreateLineMaterial();
	// set the current material
	lineMaterial.SetPass( 0 );
	GL.Begin( GL.LINES );
	GL.Color( Color(1,1,1,0.5) );
	GL.Vertex3( 0, 0, 0 );
	GL.Vertex3( 1, 0, 0 );
	GL.Vertex3( 0, 1, 0 );
	GL.Vertex3( 1, 1, 0 );
	GL.Color( Color(0,0,0,0.5) );
	GL.Vertex3( 0, 0, 0 );
	GL.Vertex3( 0, 1, 0 );
	GL.Vertex3( 1, 0, 0 );
	GL.Vertex3( 1, 1, 0 );
	GL.End();
}

Note: This class is almost always used when you need to draw a couple of lines or triangles, and don't want to deal with meshes. If you want to avoid surprises the usage pattern is this:

function OnPostRender() {
	 // Set your materials
	 GL.PushMatrix();
	 // yourMaterial.SetPass( );
	 // Draw your stuff
	 GL.PopMatrix();
}

Where at the "// Draw your stuff" you should do SetPass() on some material previously declared, which will be used for drawing. If you dont call SetPass, then you'll get basically a random material (whatever was used before) which is not good. So do it.