Formatting prefix and hyperlink

I'm struggling to make two changes to how the DOI is formatted by this package: 1) change the format of the prefix to use small caps; and 2) change the colour of the hyperlink.

1. I tried to edit the \doitext command in doi.sty to:
   \newcommand{\doitext}{\textsc{doi: }}
   but when I compile my document I get an error associated with the .bbl file:
   Missing control sequence inserted. \newblock \doi{10.1002/andp.19053221004}
2. I have set the options for the hyperref package to colour both links and citations blue but these options seem to be ignored by the doi package.

Does anyone have any suggestions? I've included my MWE file contents below. The doi.sty file is unchanged except for when I tried that one change described above.

Cheers!

\documentclass[a4paper]{article}

\usepackage{natbib}
\usepackage{hyperref}
\usepackage{doi}

\hypersetup{colorlinks=true, citecolor=blue, linkcolor=blue}

\begin{document}

	This is a test \citep{einstein}.

	\bibliographystyle{plainnat}
	\bibliography{mwe.bib}

\end{document}

@article{einstein,
	author =       "Albert Einstein",
	title =        "{Zur Elektrodynamik bewegter K{\"o}rper}. ({German})
	[{On} the electrodynamics of moving bodies]",
	journal =      "Annalen der Physik",
	volume =       "322",
	number =       "10",
	pages =        "891--921",
	year =         "1905",
	DOI =          "10.1002/andp.19053221004"
}

Start with doi.sty and reproduce the included mwe.tex and mwe.bib example. Inspect how \doitext and DOI hyperlinks are defined, then verify that small-caps prefix formatting and hyperref link colors both work without the .bbl error.】【。