changing the default color using the changes package

Originally, changes.sty calls

\definechangesauthor{\@empty}{blue}

This command calls \colorlet{Changes@Color#2}{#3} with these two values. You could do the same just with another color, so you don't need to add an author. Just write in your preamble for example:

\colorlet{Changes@Color}{red}

You can do this on a per author basis with the \definechangesauthor command. Changing Changes@Color (with \colorlet) will change the color for remarks with no associated author. A little example:

\documentclass{book} 
\usepackage{changes}
\usepackage{lipsum}

\definechangesauthor[name={Gonzalo Medina}, color=orange]{GM}
\definechangesauthor[name={John Doe}, color=blue]{JD}

\colorlet{Changes@Color}{green}

\begin{document} 

Some \added[id=GM]{new text} added in the example by GM. Some \added[id=JD]{new text} added in the example by JD. Some \added{new text} added in the example.

\end{document}