while experimenting with the sample transparency color code


\definecolor [transparentred]  [r=1,t=.5,a=1]
\definecolor [transparentblue] [b=1,t=.5,a=1]
\definecolor [solidyellow]  [y=1,t=1,a=1]

\starttext

\startTEXpage
~This is some sample text that goes behind the rectangles\hskip-80mm
\blackrule[width=18mm,height=10mm,depth=10mm,color=solidyellow]\hskip-6mm
\blackrule[width=18mm,height=20mm,depth=1mm,color=transparentred]\hskip-6mm
\blackrule[width=18mm,height=10mm,depth=10mm,color=transparentblue]\hskip-6mm
\blackrule[width=18mm,height=20mm,depth=1mm,color=solidyellow]\hskip27mm
\stopTEXpage

\stoptext



i get the intended result.


if i change the yellow t value to .99 
\definecolor [solidyellow]  [y=1,t=0.99,a=1]
some green layer pops up.


as this is all about software, there is a reason to this.
it cold be the OSX preview program too, of course.
can someone explain to me ?
:-)

.F