Conditional formatting can be applied to line charts by applying distinct colors to separate DAX measures. In the following example, a DAX measure is created that only returns the sales per order value when its value is below $600:

Internet Net Sales Per Order Below $600 = 
IF([Internet Net Sales Per Order] < 600,[Internet Net Sales Per Order],BLANK())

Using this measure and the Internet Net Sales Per Order measure on the same line chart allows for separate colors to be applied: 

For this example, a default light green can be used for the Internet Net Sales Per Order measure and red can be applied for the below $600 measure. Additionally, the below $600 line can be formatted with a slightly larger stroke width and a dotted line style via the customize series formatting options to better contrast these values. 

The line chart will appear as a single line that changes colors and styles when it goes below $600:

The stroke width, join type, line style, and marker shape formatting properties provide a wide range of options for contrasting lines beyond their colors. These additional properties are recommended to aid general comprehension and to support users who cannot easily distinguish colors.