Sorry for my bad explanation,
If a point value is below a given value ( a SplitValue) I call its color "ColorBelow".
Colors are set this way:
LineSeries.AddXY(x,y,name,color);
Look at the picture in the first post, you will see the SeriesLine has changed to blue under a value, in this example 10 degrees.
In a "quick look" a user may see if a critical value has occurred, may be too high or too low, depending on setting of the split value.