Color functional groups in chemfig syntax

Question

I want to color simple functional groups in organic molecules using the chemfig package. I've come up with a solution that achieves what I want (MWE), but I find it very cumbersome. I thought it could be possible to wrap an entire group in one single color-statement. This question about colouring the background of a group of atoms in chemfig also gives a neat graphical emphasis of a group of atoms.

Is my solution, and the solution to the question above, the easiest way to colour or mark groups of atoms (including bonds) in chemfig? Just now, I don't have a need to easily reproduce loads of coloured functional groups, but I would like to know if there are easier ways to solve this problem without predefining macros for each group.

I believe I understand that the underlying problem is the way chemfig uses TikZ nodes to draw the structures, but I can't help to think that there should be a way to define some color environment that applies to all the TikZ nodes made by chemfig.

MWE:

\documentclass{article}

\usepackage{chemfig}

\begin{document}
    \chemfig{H-C(-[2]H)(-[6]H)-C(-[2]H)(-[6]H)-{\color{blue}C}(=[1,,,,blue] {\color{blue}O})(-[7,,,,blue] {\color{blue}OH})}
\end{document}

Producing

Answer 1

Relate to this post.

\documentclass{article}
\usepackage{chemfig}
\begin{document}
    \def\RED{\gdef\printatom##1{\color{red}\ensuremath{\mathrm{##1}}}}
    \def\BLACK{\gdef\printatom##1{\color{black}\ensuremath{\mathrm{##1}}}}
    \noindent
    \chemfig{H-C(-[2]H)(-[6]H)-C(-[2]H)(-[6]H)(-[,,,,draw=none]\RED)([,,,,,red]-[,,,,black]C(=[1]O)(-[7]OH))}\BLACK \\[1cm]
    \chemfig{H-C(-[2]H)(-[6]H)-C(-[2]H)(-[6]H)(-C(=[1]O)(-[7]OH))}
\end{document}

Where

• [,,,,red] is to set the color of bonds within COOH.
• [,,,,black] is to set the color of bond between C2H5 and COOH.
• (-[,,,,draw=none]\RED) is to create an ad hoc atom to change the color.
  • you can change the color anywhere, but sometimes it may cause bad spacing/positioning.
  • (e.g. chemfig reads \RED COOH as five atoms, with the first atom having zero width.)