Bad background colors

[GoogleCodeExporter]

What steps will reproduce the problem?
1. Set a given (dark) plain background color
2. Set exactly the same RGB color as the second color for boxes (shadeTB)
3. These two colors do not match, being the background color wrong

What is the expected output? What do you see instead?
Both colors should be the same, but they are not. The background color is not 
the one defined in bgColorOne for a plain background. Surprisingly, this works 
fine for some "light" colors but it does not for some "dark" colors I want to 
use (see below).

What version of the product are you using? On what operating system?
The latest version, on Windows 7.

Please provide any additional information below.
It works well for:
\definecolor{headercol1}{RGB}{191,143,68}
\definecolor{headerfontcol}{RGB}{0,0,0}
\definecolor{boxcolor}{RGB}{255,190,90}
\definecolor{backcolor1}{RGB}{255,207,134}
\definecolor{otherbox}{RGB}{166,105,12}
\definecolor{othertext}{RGB}{13,86,166}

\begin{poster}{
grid=false,
columns=6,
headerheight=0.1\textheight,
bgColorOne=backcolor1,
%bgColorTwo=backcolor2,
%borderColor=bordercol,
headerColorOne=headercol1,
headerColorTwo=boxcolor,
headerFontColor=headerfontcol,
boxColorOne=boxcolor,
boxColorTwo=backcolor1,
headershape=roundedright,
headerfont=\Large\bf,
textborder=none,
background=plain,
headerborder=none,
boxshade=shadeTB,
headershade=plain
}

It does not work well for:
\definecolor{headercol1}{RGB}{30,62,40}
\definecolor{headerfontcol}{RGB}{255,255,255}
\definecolor{boxcolor}{RGB}{0,40,12}
\definecolor{backcolor1}{RGB}{0,120,36}
\definecolor{otherbox}{RGB}{0,109,76}
\definecolor{othertext}{RGB}{255,89,0}

\begin{poster}{
grid=false,
columns=6,
headerheight=0.1\textheight,
bgColorOne=backcolor1,
%bgColorTwo=backcolor2,
%borderColor=bordercol,
headerColorOne=headercol1,
headerColorTwo=boxcolor,
headerFontColor=headerfontcol,
boxColorOne=boxcolor,
boxColorTwo=backcolor1,
headershape=roundedright,
headerfont=\Large\bf,
textborder=none,
background=plain,
headerborder=none,
boxshade=shadeTB,
headershade=plain
}

Original issue reported on code.google.com by [email protected] on 25 Mar 2014 at 3:46

[GoogleCodeExporter]

No, no, I see. It never works well. Simply, when the colors are light, the 
difference is less visible.

Original comment by [email protected] on 26 Mar 2014 at 10:39