They appear red. Light waves from an object speeding toward you will have shorter wavelengths.

VOA: special.2009.04.29

It just checks and sees what kind of light frequencies are bouncing off the object.

只要能让它检验出,物体反射的光的频率

耶鲁公开课 - 死亡课程节选

Light waves from an object speeding away from you will stretch into longer wavelengths.

VOA: special.2009.04.29