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 ?

:-)