The issue is the property "MarkerEdgeColor".  In Matlab, this accepts string color values (e.g., 'k' or 'r'), RGB color values (e.g., [1, 0, 0]), and the special string value "auto".

In the case of "auto", Matlab uses the value of the "Color" property for the value of "MarkerEdgeColor".  The issue is that Octave only implements an actual color specification.  Re-titling the bug to reflect that.

In the meantime, there is a simple workaround of using a LineSpecification argument to stem.  This is Matlab-compatible so when coded this way will work in either language.

Instead of

stem (1:100, 'color', 'k')

try

stem (1:100, 'k')

That would be part of the same changeset.  Matlab has changed the implementation of various stem properties as well as adding new ones.  It all needs to be addressed at once.

Properties that are missing in Octave:

ColorMode
LineStyleMode
MarkerMode
XDataMode
DataTipTemplate

When using stem() with an 'color' argument, like this 

  stem([1:100],'color','k')

or

  stem([1:100],'color',[1,1,1]*2/3)

the color is applied only to the vertical lines and not to the marker symbols.

In Matlab, the color is applied to the lines and the marker symbols. I'd also prefer this behavior.

I've tested this on GNU/Linux with qt, fltk and gnuplot on octave 4.4.1, and octave-development (version 6.0.0) compiled from source on 2019-09-27