When the singular values plot is generated for a list of systems, the colors of the lines should be different for each system. This doesn't seem to be happening.

Example:

import control as ct
sys1 = ct.rss(4, 2, 2, strictly_proper=True)
sys2 = ct.rss(4, 2, 2, strictly_proper=True)
ct.singular_values_plot([sys1, sys2])

generates

The following code works correctly in terms of colors:

ct.singular_values_plot(sys1)
ct.singular_values_plot(sys2)

Note that the range of frequencies is different for each system, which is the reason you want to be able to call them as a list.