It's not easy to change the line color at the right place (between points where it crosses the target). But this might work just as well.
Date and score are in columns A and B. Column C is for better than target; the formula in C2 and filled down is:
=IF(B2>$F$2,B2,NA())
Column D is for worse than target; the formula in D2 and filled down is:
=IF(B2>$F$2,B2,NA())
If performance exactly equals target then neither column has a value.
Create a line chart of date vs the three score columns. Score (column B) uses gray lines and markers, Above and Below use no lines, Above (column C) uses blue markers, Below (column D) uses orange markers. If the score is exactly equal to target, then the gray marker will appear.
The blue and orange are used instead of green and red so that individuals with color vision deficiencies (8% of adult males) can distinguish the colors. Most mammals besides primates, too.
How did I get the darker line at the target (Y=100)? I formatted the Y axis so that the horizontal axis crosses at 100. The darker line is the X axis. I also formatted the X axis so it had no tick marks, because those looked weird up in the middle of the data.