I tried Richi's way by setting LIGHT and red/blue/green. Works fine for all panel-elements. You can make the panel brighter by pressing "1" or darker by pressing "2".

Code:
BMAP* panel_png = "a7Cube.png"; 
var vred;
var vblue;
var vgreen;

PANEL* yourPanel =
{
	digits(300,0,"red: %.0f",*,1,vred); 
 	digits(300,20,"blue: %.0f",*,1,vblue);
	digits(300,40,"green: %.0f",*,1,vgreen);
	
	bmap = panel_png;
  	flags = SHOW;
}

function main()
{
	level_load("");
	set(yourPanel,LIGHT);
	
	vec_set(yourPanel.blue,vector(128,128,128));
	
			
	while(1)
	{
	
		if (key_1)
		{		
			yourPanel.blue=clamp(yourPanel.blue+5*time_step,0,255);
			yourPanel.red=clamp(yourPanel.red+5*time_step,0,255);
			yourPanel.green=clamp(yourPanel.green+5*time_step,0,255);
		}
		
		if (key_2)
		{		
			yourPanel.blue=clamp(yourPanel.blue-5*time_step,0,255);
			yourPanel.red=clamp(yourPanel.red-5*time_step,0,255);
			yourPanel.green=clamp(yourPanel.green-5*time_step,0,255);
		}
			
		vred=yourPanel.red;
		vblue=yourPanel.blue;
		vgreen=yourPanel.green;
		
		wait(1);
		
	}
}