It isn't clear what specific document class you are using, but I am able to get (for example) red equations by using the color
package and setting the color within the equation
environment.
\ifPreview
can be used as shown to turn coloring off when not in preview mode. The package should be disabled by removing the active
option to \usepackage
so as to keep \ifPreview
defined (as opposed to disabling the entire package, which would break things).
A MWE:
\documentclass[12pt]{article}
\usepackage[active,tightpage,displaymath,textmath]{preview}
\setlength\PreviewBorder{2pt}
\PreviewEnvironment{equation*}
\makeatletter\renewcommand\@eqnnum{}\makeatother
\usepackage{color}
\usepackage{lipsum}
\begin{document}
\lipsum[1]
\begin{equation}
\ifPreview\color{red}\fi
a^2 + b^2 = \frac{1}{\sqrt{\pi r^3}}
\end{equation}
\lipsum[2]
\end{document}
Gives previews as:

But when disabled, appears as:
