[pdftex] background color changes from MiKTeX version 2.7 to 2.9

[Peter Zimmermann]

Dear pdftex experts,

when pdftexing the file

\documentclass{article}
\usepackage{color,graphicx}
%% \pdfminorversion=3
\begin{document}
\pagecolor{blue}
\null
\pagebreak
\includegraphics{pict.png}
\end{document} 

with a multicolor picture pict.png the background color changes when running the same file
on a (Windows XP) machine with MiKTeX 2.7 and MiKTeX 2.9. This is visible with
Adobe Acrobat/Reader (version >= 7.0), not with SumatraPDF.
In the 2.9 version the blue background looked shaded on page 2 (not so with 2.7).
I have verified that all loaded files have the same version except:
pdfTeX, Version 3.1415926-1.40.8-beta-20080627 (MiKTeX 2.7) versus
pdfTeX, Version 3.1415926-2.4-1.40.13 (MiKTeX 2.9)

and 
LaTeX2e <2005/12/01> (2.7) versus LaTeX2e <2011/06/27> (2.9) 

The picture is a screenshot clip from a vignetting chart of a zoom-objective (reproduceable 
with other clips; clip from page http://www.photozone.de/nikon--nikkor-aps-c-lens-tests//242-
nikkor-af-s-18-200mm-f35-56-g-if-ed-vr-ii-dx-review--test-report?start=1).

If I use \pdfminorversion=3 with 2.9 the problem is gone (from \pdfminorversion=4 on
the problem occurs). What has changed in pdftex and why?

Many thanks for your effort
Peter Zimmermann