Markings that have this rule must be darker than anything they cover. It should generally be close in hue and saturation to the base coat or what it's covering.
In the case of a black base coat (onyx, midnight, black, etc), if the chosen base colour is as dark as it can legally be and making markings darker would cause them to be #000 or indistinct from the lineart, they should instead be the same colour as the base coat. In these cases, one should make sure the marking is still distinct in its placement; for instance, you should not have point be the same colour as the base unless there are some markings peeking through beneath that make it clear that it's still that marking. If it's impossible to make that clear, one should make their base coat slightly lighter to accommodate.