How to define color PANTONE 300 c in LaTeX

If you are using pdflatex or lualatex, you can use the colorspace package:

You can give the alternate colour in either RGB, CMYK, or Lab.

However, I notice that colorspaceconverts the RGB values to CMYK, so maybe it's best to stick with CMYK or Lab alternate colours. (This is noted in the manual.) In fact I think it's definitely better not to use the RGB model, since the conversion to CMYK is done using the simple xcolor formulas which make no attempt to use colour management. So an RGB alternate colour will look quite different (as shown in the example below).

\documentclass{article}

\usepackage[illuminant=d50]{colorspace}
\definespotcolor{PANTONE 300 C (CMYK)}{PANTONE 300 C CMYK}{0.99,0.5,0.0,0.0}
\definespotcolor{PANTONE 300 C (RGB)}{PANTONE 300 C RGB}[RGB]{0 94 184}
\definespotcolor{PANTONE 300 C (Lab)}{PANTONE 300 C Lab}[alt=lab]{35.78,-9.69,-62.01/0.99,0.5,0.0,0.0}

\begin{document}

\textcolor{PANTONE 300 C (CMYK)}{PANTONE 300 C (alt = CMYK: 0.99, 0.5, 0.0, 0.0)}

\textcolor{PANTONE 300 C (RGB)}{PANTONE 300 C (alt = RGB: 0, 94, 184)}

\textcolor{PANTONE 300 C (Lab)}{PANTONE 300 C (alt = Lab: 35.78, -9.69, -62.01; illuminant = d50)}

\end{document}

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If you are using xelatex, you can use the xespotcolor package. This package isn't as feature filled as the colorspace package.

\documentclass{article}

\usepackage{xespotcolor}

\NewSpotColorSpace{PANTONE}
\AddSpotColor{PANTONE}{PANTONE300C}{PANTONE\SpotSpace 300\SpotSpace C}{0.99 0.5 0 0}
\SetPageColorSpace{PANTONE}
\definecolor{PANTONE 300 C}{spotcolor}{PANTONE300C, 1.0}

\begin{document}

\textcolor{PANTONE 300 C}{PANTONE 300 C (alt = CMYK: 0.99, 0.5, 0.0, 0.0)}

\end{document}

\documentclass{book}
\usepackage{xcolor}
\definecolor{newcolor}{cmyk}{1,.56,0,.03} % PANTONE 300 c
% or ...
%       \definecolor{newcolor}{RGB}{0,94,184} 
%       \definecolor{newcolor}{HTML}{005EB8} 

\begin{document}
\textcolor{newcolor}{Sample text}
\end{document}