I have to do a chocolate cake with chocolate buttercream crumb coat. Client wants thick buttercream under the fondant.

Will I have a problem with brown buttercream under white fondant?

I have never had an issue with a thin layer of chocolate ganache under white fondant, but a thick layer of chocolate buttercream with a normal thickness of fondant will have a very slight tint underneath. If I were you I would make time to do a test run with a 4" round to determine if you get the result you desire.

You can do chocolate buttercream under the white fondant but measure twice - apply once! Make double sure your buttercream has crusted (I even put my in the freezer for about 10 minutes). If you make a boo boo and have to take off the fondant you can gently wipe off the chocolate with a damp towel, knead, re-roll and try again.