Color schema for Gray and Plain QtCurve (0.69 version required) style. The style itself can be found with same name.

TIP. If you have littlebit blueish monitor, set the gray color your self littlebit warmer. Place hue to 60 on colors and set saturation between 0-20 (or more) depending your monitor, to get pure gray color. If your monitor is warmer, then place hue to 240 and set saturation as well.