Reducing the distance in textbf environment

I want to introduce a notation for concatenating vectors in a manuscript that I represent vectors with bold characters, $\textbf{x}$. The problem is defining it like $\textbf{x}:\textbf{y}$ introduces an extra distance between two characters which is far more than $\textbf{x:y}$. On the other hand the latter is not what I am looking for since I need non-bold subscripts for vectors and I need to use two $\textbf$ environments. Any suggestions?

\articleclass{document}
\begin{document}
This is bad concatenation $\textbf{x}:\textbf{y}$
This is good concatenation $\textbf{x:y}$
\end{document}

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Enclose the : itself in braces (to prevent it from being treated as a binary operator, which causes the space). –  Paul Gessler Jul 4 at 13:20
: is a considered a relation symbol; $\mathbf{x}{:}\mathbf{y}$ is what you need. –  egreg Jul 4 at 13:20

\documentclass{article}

%uncomment to make : act like {:} by default.
%\DeclareMathSymbol{:}{\mathord}{operators}{"3A}

\begin{document}

$\mathbf{x}:\mathbf{y}$

$\mathbf{x}{:}\mathbf{y}$

\end{document}

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I'm sure \mathbf should be used; or the symbols could turn out to be in bold italic, if the formula is inside a theorem statement. –  egreg Jul 4 at 13:24
@egreg Oh that is a good point! Thanks –  Naji Jul 4 at 13:25
@Naji If you only need : in that context and never as a relation symbol, I'm sure David will be glad to add a way for inputting : directly. –  egreg Jul 4 at 13:27
@egreg done: -) –  David Carlisle Jul 4 at 13:32
@Naji The whole point of bm is to give the bold version of a character so if it was in math italic it gives bold math italic. If you want upright bold then \mathbf will give that –  David Carlisle Jul 4 at 13:33