Bug report

I adapted a stylesheet from this official source

when I uncomment line 148:
boxplot.medianprops.color : 'C1'

I do get a warning. I think this warning is not specific to line 148 but has something to do with either the boxplot styles or the color argument.

Code for reproduction
from matplotlib import pyplot as plt
plt.style.use('custom_style.mplstyle')

Actual outcome

UserWarning: Bad val "'C1'" on line #148 Key boxplot.medianprops.color: 'C1' does not look like a color arg

Matplotlib version

• Operating system: Windows 7 enterprise
• Matplotlib version: 2.1.2
• Matplotlib backend : tried Agg, TkAgg, Qt4Agg
• Python version: 3.6
• Jupyter version (if applicable): 1.0.0
• Other libraries:

i used conda defaults channel for installing matplotlib.