For the standard classes titlesec would be my choice. Here's an example using titlesec and the standard formatting/spacing, except that the font is changed to Helvetica (Arial has been derived from it) and the font color for demonstration. Adjust the \titleformat arguments to achieve what you desire.
\documentclass{report}
\usepackage[english]{babel}
\usepackage{blindtext}
\usepackage{mathptmx}% Times Roman font
\usepackage[scaled=.90]{helvet}% Helvetica, served as a model for arial
\usepackage{xcolor}
\usepackage{titlesec}
\titleformat{\chapter}[display]
{\normalfont\sffamily\huge\bfseries\color{blue}}
{\chaptertitlename\ \thechapter}{20pt}{\Huge}
\titleformat{\section}
{\normalfont\sffamily\Large\bfseries\color{cyan}}
{\thesection}{1em}{}
\begin{document}
\Blinddocument
\end{document}

With KOMA-Script classes it's easy as many other issues, just use \setkomafont
and \addtokomafont
. Here's the example:
\documentclass{scrreprt}
\usepackage[english]{babel}
\usepackage{blindtext}
\usepackage{mathptmx}% Times Roman font
\usepackage[scaled=.90]{helvet}% Helvetica, served as a model for arial
\usepackage{xcolor}
\usepackage{titlesec}
\setkomafont{chapter}{\normalfont\huge\sffamily\bfseries\color{blue}}
\addtokomafont{section}{\color{cyan}}
\begin{document}
\Blinddocument
\end{document}
