I have found something... the Email Customizer uses variables and it's own color palette... the color palette is defined by my custom colors, however it maybe possible that through incompatibility of both add-ons it does not work together and it uses default settings of the Email Customizer. So, I will circumvent it by deleting the @footerBackgroundColor style-variable from EMail Customizer and write solid color numbers into the styling. Let me test it.