to make sure the plot and the histogram have the same colour, my suggestion is that you fix the colour for the plot and for the best fit line.
If you look at the example here http://matplotlib.org/1.2.1/examples/pylab_examples/histogram_demo.html
and then at the python documentation for pyplot http://matplotlib.org/1.2.1/api/pyplot_api.html?highlight=hist#matplotlib.pyplot.hist
the matplotlib.pyplot.hist method has a kwarg color that allows you to choose the colour you want for the histogram. In the example they set facecolor='green'
Then for the best fit line, you can choose to plot it in the same colour. I would need to see the code to give more precise indications. However if we go back to the example here the line properties are set:
l = plt.plot(bins, y, 'r--', linewidth=1)
so if we wanted the fit line to be green like the rest of the histogram we would use:
l = plt.plot(bins, y, 'r--', linewidth=1, color = 'green')
Hope this helps, can't give you more specific tips if you don't post any lines of code.