I think for aerial imagery, the color cannot get changed, it don't support this parameter.
Our map API "ColorMappings" can dynamic render target color to the other specified color, so it can make the image looks light, but I test it and found it don't works for your scenario, because there are too many different colors in an aerial imagery, it works well in road map.
And finally I found if you set this the property "Transparency", it will looks like a little light.
bingMapsLayer.Transparency = 200;
Wish that's helpful.