I wrote:

mpl.rcParams['lines.color'] = 'r'
...the line is still blue.
BR answers:

Plot() doesn't use lines.color.  I don't remember the exact name, but it uses an rcparam for color cycling.  Just change make the list of colors be just 'r'.
Of course I found that myself some time after reporting this "bug"... Sorry.
The parameter is "axes.color_cycle"

Jerzy K.