Why does \color{blue} differ from \color{Blue}?

Question

    \documentclass[dvipsnames]{article}
    \usepackage{xcolor}
    \begin{document}
    {\color{Blue}\bf Blue} versus {\color{blue}\bf blue}.
    \end{document}

Why does one of them differ from the other one?

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EDIT 1

1. I still wonder why the color is distinguished by capitalization :-)
2. Why did the package author choose dvipsnames as the option name? I think this name potentially makes us obtain a wrong impression from it that is usually used to describe the driver? In fact both pdflatex and latex can use it, right?

Answer 1

blue is a rgb color and Blue is defined as a cmyk color and handled by a different driver for your output. If you want to be sure, that you use the same driver, then specify one:

\documentclass[dvipsnames]{article}
\usepackage[rgb]{xcolor}

\begin{document}

{\color{Blue}\bf Blue} versus {\color{blue}\bf blue}.

\end{document}

instead of [rgb] you can also use [cmyk]

Answer 2

Because that's how the colours are defined :)

Colour names are case sensitive, and red, green, blue are the ‘standard’ (1,0,0), (0,1,0), (0,0,1) RGB colours respectively.

In section 4 of the xcolor manual (pg. 38 for v2.11) the list of other colour names is given; under the dvipsnames section you can see that there are many more colours defined with initial-uppercase names. Personally, I find those Red, Blue, and (especially) Green colours to be much more attractive as they are a little more muted.