Warbird Information Exchange

I've just been given an ASI made by the US Gauge Co which is calibrated 0-190 on the outer scale and 200-430.

On the face it appears to be marked wit a P/N AD-5363-P and on the rear the MFG P/N is given as AW-23/4-20-B.

All that's just background as the real reason for the post is this.

The face of the gauge has the usual white numerals and graduations but the background to them isn't the normal black. It's a faded brick red colour.

Can anyone shed light on the use of that shade as a background?

I've never seen anything other than black before (excepting white faces with black markings for some ammeter and voltmeter units)

tia