There are a couple ways of looking at it.
"Marker = Het" might be true. If you have a big clutch of pos hets and you breed all the markers they MIGHT all prove out....to my knowledge no one has done this to prove that it's true?
However we have proven that "Het != Marker". Meaning if you have that same clutch of pos het and you breed all the non-markers some will most likely prove to be hets. (or another way to look at it is a clutch of all 100% hets...some will have markers some will not, even though all are hets)
While I'm not a big fan of saying the former is gospel truth....I'd lean towards picking out a "marker" het in a clutch of pos hets myself.
That being said the OP's question:
To some degree clowns have "markers"....in that alot of times het clowns are lightly colored and have high white on their sides....basically the prettiest looking ones might be hets 
Hope that helps!
>>I respect your opinion BUT pessimistic, not!!!
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>>Is it black or white, het-pied markers fall more into the grey zone, don't they !?!
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>>If all Het-Pieds had the marker then yes, it would be a full marker, the simple fact that some 100% Het-Pieds do not have the said marker puts an end to the debate, doesn't it?
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>>The Het-Pied marker phenom was started by those selling possible het-Pieds in order to corner the Het-Pied market vrs other possible het-pieds with no marker.
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>>In my eyes, its a 100% marker or it isn't.
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>>I'd like to hear a concrete theory on why Het-Pied markers should be accepted.
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John Dague
JD Constriction
www.jdconstriction.com