I read gauges like I read a speedometer -- the angle of the needle gives me most of the information. It hasn't mattered at all what color the face is.
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As my white face gauges give up the ghost I am switching to black. For me the black face gauges are easier to read at night or in low vis as what stands out are the lines and numbers. At night my white face gauges when hit with the HID are bright enough to not only obscure the lines due to the glow but also mess up my night vision for a few minutes. The blacks don't do that.
For rental gear if someone is confused by the color of the SPG's face they probably should not be using scuba gear in the first place.
Does anyone make SPG's with different orientations? For example I have a black 2" spg on my AL40 that's slung on my left side. I don't have it lollipoped, I have it pointing down along thank. But this forces me to read the gauge from the side.