I deleted Spinebot's reply which unfortunately does not apply to spine-csharp 4.2 and was incorrect for 4.3-beta.
Which version of the spine-csharp runtime are you using?
If you're on 4.2, the color properties are separate slot.R, .G, .B, .A properties:
EsotericSoftware/spine-runtimesblob/4.2/spine-csharp/src/Slot.cs#L98-L107
And .R2, .G2, .B2 for the dark slot color (for tint-black).
If you're on 4.3-beta, you can now set it via GetColor and SetColor, but you need to set it at the slot pose, not the slot, so slot.AppliedPose or slot.Pose. So you can use slot.AppliedPose.GetColor() and slot.AppliedPose.SetColor().
