How should I interpret markerplots with colours indicated as below zero on the markerplot scale?

[SMcCartan]

Hi, I've just started playing around with Citrus over the past month.
Can you give me an idea on how I would interpret data where coloured clusters correspond to a minus number on the accompanying scale in marker plots? e.g. my marker plot is green and blue but the green is -0.35 on the scale.
Is is the case that this marker is too low overall? In which case should I remove markers like this from my analysis?
Or is the marker expression in the cluster low as a consequence of scaling?
I have read problem #84 and the related links but am none the wiser on this issue.
I'd be grateful for any info, or in being pointed towards refs that can build up my knowledge.
(Background biology, but trying and willing to get to grips with bioinfomatics and R).
Cheers, Sheila.