\nopagecolor of pdftex.def (and LuaTeX) disables the page color, it does not use any other color like white. Compare and look closely at the cropped images:
\documentclass{article}
\usepackage{xcolor}
\begin{document}
% Page 1
\nopagecolor
\verb|\nopagecolor|
\newpage
% Page 2
\pagecolor{white}
\verb|\pagecolor{white}|
\end{document}


The second page has a white background.
The first page does not have a background, which can also be verified by the page stream operators for the first page with some added comments:
stream
0 g 0 G % set black as fill and stroke color
0 g 0 G % set black as fill and stroke color
0 g 0 G % set black as fill and stroke color
BT % begin text
/F15 9.9626 Tf 148.712 657.235 Td [(\134nopagecolor)]TJ % Write "\nopagecolor"
0 g 0 G % set black as fill and stroke color
0 g 0 G % set black as fill and stroke color
ET % end text
endstream
The white background of GIMP adds the white background. I have not seen an option to disable it (version 2.8). If the white color is only used for the background, then option "Colors → Color to Alpha" can convert white back to the alpha transparency channel.
\nopagecoloror if background is not explicitly set (which is transparent by default). – alwaysask Aug 07 '16 at 20:58