Similar to solutions in Visualization in LaTeX of hamming distance, and Problem with the alignment of characters, you could use the literate
command to define a style that is to be applied to each digit. Below I included a color for the .
but commented out the color for the ,
.

As there might be a period used outside of a number context, I have defined different styles based on the assumption that any period used in a number will have a digit following it.
\documentclass[border=2pt]{standalone}
\usepackage{listings}
\usepackage{xcolor}
\newcommand*{\FormatDigit}[1]{\textcolor{blue}{#1}}
\lstdefinestyle{FormattedNumber}{%
literate={0}{{\FormatDigit{0}}}{1}%
{1}{{\FormatDigit{1}}}{1}%
{2}{{\FormatDigit{2}}}{1}%
{3}{{\FormatDigit{3}}}{1}%
{4}{{\FormatDigit{4}}}{1}%
{5}{{\FormatDigit{5}}}{1}%
{6}{{\FormatDigit{6}}}{1}%
{7}{{\FormatDigit{7}}}{1}%
{8}{{\FormatDigit{8}}}{1}%
{9}{{\FormatDigit{9}}}{1}%
{.0}{{\FormatDigit{.0}}}{2}% Following is to ensure that only periods
{.1}{{\FormatDigit{.1}}}{2}% followed by a digit are changed.
{.2}{{\FormatDigit{.2}}}{2}%
{.3}{{\FormatDigit{.3}}}{2}%
{.4}{{\FormatDigit{.4}}}{2}%
{.5}{{\FormatDigit{.5}}}{2}%
{.6}{{\FormatDigit{.6}}}{2}%
{.7}{{\FormatDigit{.7}}}{2}%
{.8}{{\FormatDigit{.8}}}{2}%
{.9}{{\FormatDigit{.9}}}{2}%
%{,}{{\FormatDigit{,}}{1}% depends if you want the "," in color
{\ }{{ }}{1}% handle the space
,
basicstyle=\ttfamily,% Optional to use this
}
\newcommand{\FormattedNumber}[1]{%
\lstinline[style=FormattedNumber]{#1}%
}
\begin{document}
\FormattedNumber{a.vector([3/5,4,0.4566])}
\end{document}
\documentclass
so that those trying to help don't have to recreate it.