Incompatible color definition when using tikz with svgraybox (from svmono)

With the following code I get the warning “Incompatible color definition”. When I remove the package tikz, the warning disappears. But I need this package. Can I avoid the warning?
I have to use the class svmono from
https://www.springer.com/engineering?SGWID=0-175-6-1381428-0

Thanks!

RequirePackage{amsmath}            % Instead of usepackage{amsmath} in order to avoid Warning: Unable to redefine math accent vec
documentclass[graybox,envcountchap,sectrefs]{svmono}

usepackage{newtxtext,newtxmath}    % Instead of mathptmx, since with mathptmx Warning: there are no bold math fonts 
usepackage{graphicx}
usepackage{tikz}
begin{document}
begin{svgraybox}
Hello.
end{svgraybox}
end{document}