Another resource I can recommend is this:
Color Mixing Guide
Especially the graph is helpful.
In your case:
Result is black which is in the center.
Gleam is shadow red which is to the up and right of that.
This means that you likely came from the lower left which is the blueish area like azure, slate, teal, turquoise.
All colors involved are dark colors with low luminance.
So it likely was one of the darkest blue variants or perhaps dark grey.
If you encountered a random mutation it can be anything of the dark colors in the center.
We do not know how the mutation paths look exactly which makes it hard to predict precisely over which colors the mutation will jump.