In [1]: plot([1, 2], color=(.75, .25, .25)) # <- normal reddish color

Select the parameter editor (green tick) and directly click ok. The line becomes black. In fact, if you check the Curves panel, the color editor gives #000000 as color. A quick look at formlayout.py shows that to_qcolor simply casts the color attribute to a string (so "(0.75, 0.25, 0.25)" in our case) and parses it as hex, giving up and returning black if it doesn't look like a hex RGB.