Markings that have this rule must be lighter 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 white base coat (leucistic, snow, white, etc), if the chosen base colour is as light as it can legally be and making markings lighter would cause them to be #fff, 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 roan or another white spotting marking be the same colour as the base unless there are some markings peeking through 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 darker to accommodate.