Open the aircraft's folder.

Open the panel folder.

Open the panel.cfg notepad file.

Go down to the bottom of the panel.cfg file.

You should see something like this:

[Color]
Day=255,255,255
Night=255,255,255
Luminous=190,255,190

The panel lights are controlled by the Night line.  The Luminous line controls the backlighting color of the gauges.  

Do this:

Cross off the Night line like this:
//Night=255,255,255

Put two slashes in front of the line.  The //'s render the line unreadable, but you still have the line there for reference or to restore back to original parameters.

Re-enter the Night line values like this, with all 255's:

Night=255,255,255

That should give maximum brightness to the panel.  If you want to reduce the brightness, enter all three values the same, for example, 200,200,200 will give a shade of gray that will reduce the brightness.

(If that doesn't work, delete the line you just entered, remove the slashes, and try the same operation on the "Luminous" line, altho I'm pretty sure it's the Night line you want to change.)

The numbers give the values of red, green, and blue.  All 255's show up white, all 0's would be black.  When all numbers are the same, you'll get shades of gray.  

If you want to change the backlighting color of the gauges, you can determine the color values you want with a photo editor.  You can also determine color values in Excel or Word with the color palettes therein.  For example, this combination: 190,255,190, will give a nice shade of pastel green.  

It's been a long time since I've modified panels, but I think I got this right.  Corrections are certainly welcome.

Just remember, you can't mess anything up, you're only changing numbers in a notepad file, and they can be easily changed back to what they were.