Short Summary
you have 2 color layer, if you overlap them, you will create color disaster
because color 1 + color 2 = completely new color 3( will not show on legend )
if new color 3 is not show on legend, user will never know what that means.
yellow(layer 1) + blue(layer 2) = (overlap area) green
green is neither on layer 1's legend, nor on layer 2's legend, so user never know green means what
Long Story Details
Here is sample of 2 color layer overlap
layer 1 : Health Professional Shortage Areas (HPSAs)
layer 2 : Medically Underserved Areas/Populations (MUA/Ps)
This color mess is disater.
https://geocarenavigator.hrsa.gov/?page=Home
To avoid color overlap disaster, you should use hash line
same 2 layers with hash line, you can see where they overlap
https://transparentgov.net/json2tree/datahub.io/uds/uds_data.html?_center_zoom=9&_center_lat=33.31934619365885&_center_long=-116.60984309491538&widgetopacity=0.9&tooltype=off_select2&showServiceArea=hide&patientsIncluded=34&showHealthCenter=show