Separating Trendlines by Color

To separate all the linear trendlines and plot them in the same color as main data they describe, navigate to the Marks menu, and select Separate trendlines by color.

separate trendlines

Note that the trendlines of visual match the color of the line dimension they describe.

Line chart for world population: sum(population) on Y axis, year on X axis, country generating different upward trending lines, calculated trend lines as dashed lines, matching colors
Line Chart with Trendlines Showing