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

Question

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}

Answer 1

Now I could resolve it. The solution was replacing in svmono.cls itself the command \Requirepackage{color} by \Requirepackage{xcolor}.