In the case of the ambulance and police car, it's more a question of brightness.

警车和救护车上之所以用红灯，大多是因为红灯亮度大。

耶鲁公开课 - 文学理论导论课程节选

You can--Degrees of pushing, degrees of heat, degrees of brightness.

你能够感受到按压的程度,受热的程度,明亮的程度

耶鲁公开课 - 心理学导论课程节选