From memory, the focusing light is always the same regardless of the grade you select; it's only during exposure that you see the changes in colour between grades. If the focusing (green) lamp seems dim, the wrong bulb might be fitted, such as 150W for green and 300W for blue.

Although the program switch does change the intensity of the green light, the changes are small - in the order of 10% per step. The program switch is used to balance the relative intensities of the lamps so that when you select a grade, the lamps correctly expose for that grade.