default to black, not the colour representing strong association, when c...

...olouring pointers on Phenotype Karyotype view

NB - I don't know if any other karyotype (or other) views use this same bit of code, but we've had multiple requests to display phenotype associations with unknown p-values in a neutral colour, rather than confusingly labelling them as strongly associated by colouring them red.

If there are other views that use this gradient code, then this patch will affect them too, so please investigate this before merging!