# How to overwrite siunitx's binary prefixes

Is there something special that needs to be done to overwrite a binary prefix using the siunitx package? Overriding the SI prefixes is no problem. The MWE with \sisetup{binary-units=true} yields:

but with \sisetup{binary-units=false} I obtain:

Note that I was able to easily redefine \kilo, but am unable to redefine \bit:

## MWE:

\documentclass{article}
\usepackage{siunitx}
\usepackage{xcolor}

%% http://tex.stackexchange.com/questions/88848/how-do-i-typeset-units-like-mb-gb-megabytes-gigabytes
\sisetup{binary-units=true}%

\DeclareSIUnit\bit{\textcolor{red}{bit}}%
\DeclareSIUnit\kilo{\textcolor{red}{k}}%

\begin{document}
\si{\bit}
\si{\kilo}
\end{document}

-

As the units here are optionally loaded, and as there is a need to allow compatibility with v1, the actual loading is done at the start of the document. Thus you need

\documentclass{article}
\usepackage{siunitx}
\usepackage{xcolor}

\sisetup{binary-units=true}%
\AtBeginDocument{
\DeclareSIUnit\bit{\textcolor{red}{bit}}%
\DeclareSIUnit\kilo{\textcolor{red}{k}}%
}
\begin{document}
\si{\bit}
\si{\kilo}
\end{document}

-