Lines on a chart are based on the series
Each line will have one colour
You could split your data for each server into two series (with and without prediction).
The issue with that might be that there will be a break between the end of one line and the start of the next.
Also, a series with only one data point becomes a dot rather than a line.
To overcome these, you could duplicate the last non-prediction entry as a prediction entry.
In order for the lines to have the same colour, you could set the series colours with formatting options.
In order to make some of the lines dotted / dashed, you could use CSS.