In all honesty, (huge controversy coming up now) perhaps the photograph in Mark Hill's book is distorting the colour, but I think that fish he shows as cobalt, to my eye at least, is not cobalt blue
or at least the base glass used for it is not cobalt blue.
It might have cobalt blue in the wings but even then I would have expected it to 'show' as cobalt in the photograph, in the same way as my fish vase does because my fish vase contains the cobalt blue in the wings.
I've photographed my fish in various conditions under various types of lighting bulbs and each and every time, it picks up as cobalt, never that jewel deep turquoise that it actually displays as to the naked eye, despite having the cobalt swirls in it.
m