Theory :

This one was published long time ago using Kalman filter name (among other publications, you can check this one too : Kalman filter - a version for mt4). Kept the name and made it use what metatrader 5 can do now : being applied to another indicator data

PS: the color of the indicator is nor based on the slope of the indicator, but on the internal "velocity" that is calculated in the process of calculating this indicator

Usage :

Apply it to price or any other indicator you wish to use and see what the  filter shows you. You can use color changes as signals