You could simply use \AtBeginBibliography and \AtEveryBibitem to set the code for colorizing the bibliography entries only of the important category.
P.S. = Since you didn't add an example of your .bib file, I've used biblatex-examples.bib.
\documentclass{article}
\usepackage[utf8]{inputenc}
\usepackage[backend=biber,
isbn=true,
giveninits=true,
style=numeric,
maxnames=99,
sorting=ydnt,
defernumbers=true,
autocite=superscript]{biblatex}
\defbibheading{bibliography}[\refname]{}
\addbibresource{biblatex-examples.bib}
\renewbibmacro{in:}{}
\usepackage[usenames,dvipsnames]{xcolor}
\DeclareBibliographyCategory{important}
\addtocategory{important}{knuth:ct:a}
\addtocategory{important}{knuth:ct:c}
\AtBeginBibliography{%
\DeclareFieldFormat{labelnumberwidth}{\ifcategory{important}%
{\color{orange}\mkbibbrackets{#1}}%
{\mkbibbrackets{#1}}%
}}
\AtEveryBibitem{\ifcategory{important}
{\color{orange}}
{}}
\begin{document}
\section{Main text}
\cite{knuth:ct}
\cite{knuth:ct:a}
\cite{knuth:ct:b}
\cite{knuth:ct:c}
\cite{companion}
\section{Bibliography}
\printbibliography
\end{document}

Edit:
The \ifcategory command has this syntax:
\ifcategory{hcategoryi}{htruei}{hfalsei}
similar to may other \ifs commands, see biblatex manual para. 4.6.2 "Stand-alone Tests".
Within htruei or hfalsei you can nest as many \ifcategorys (or other \ifs) you want, you only have to pay attention matching all the braces.
Here is an example with three different colors for three different categories:
\documentclass{article}
\usepackage[utf8]{inputenc}
\usepackage[backend=biber,
isbn=true,
giveninits=true,
style=numeric,
maxnames=99,
sorting=ydnt,
defernumbers=true,
autocite=superscript]{biblatex}
\defbibheading{bibliography}[\refname]{}
\addbibresource{biblatex-examples.bib}
\renewbibmacro{in:}{}
\usepackage[usenames,dvipsnames]{xcolor}
\DeclareBibliographyCategory{important}
\addtocategory{important}{knuth:ct:a}
\addtocategory{important}{knuth:ct:c}
\DeclareBibliographyCategory{awards}
\addtocategory{awards}{knuth:ct:b}
\DeclareBibliographyCategory{ducks}
\addtocategory{ducks}{companion}
\AtBeginBibliography{%
\DeclareFieldFormat{labelnumberwidth}%
{\ifcategory{important}% if
{\color{orange}\mkbibbrackets{#1}}% then
{\ifcategory{awards}% else if
{\color{blue}\mkbibbrackets{#1}}% then
{\ifcategory{ducks}% else if
{\color{green}\mkbibbrackets{#1}}% then
{{\mkbibbrackets{#1}}%else
}% end if
}% end if
}% end if
}}
\AtEveryBibitem%
{\ifcategory{important}% if
{\color{orange}}% then
{\ifcategory{awards}% else if
{\color{blue}}% then
{\ifcategory{ducks}% else if
{\color{green}}% then
{}%else
}% end if
}% end if
}% end if
\begin{document}
\section{Main text}
\cite{knuth:ct}
\cite{knuth:ct:a}
\cite{knuth:ct:b}
\cite{knuth:ct:c}
\cite{companion}
\section{Bibliography}
\printbibliography
\end{document}

\documentclassand ending with\end{document}. – Bobyandbob Apr 28 '17 at 11:53\newcommand{\citeColored}[2]{\hypersetup{citecolor=#1}\cite{#2}\hypersetup{citecolor=blue}} and in text: \citeColored{yellow}{eg}. Please always add a MWE or MWEB. – Bobyandbob Apr 28 '17 at 12:01\defbibenvironment{bibliography}and\printfieldbe helfpful here? – Bobyandbob Apr 29 '17 at 10:37