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Hi Shelley,
You are certainly right about IC 2602 -- it is one of the very best open clusters for binoculars. It is interesting that you noted Spica's colour but didn't see colour in any of the stars in IC 2602 -- which are about as blue as they can be to our vision. There are three stars in the cluster with B-V values that are "more blue" than -0.13 -- the same as Spica. Theta itself is -0.20 (which is exceptionally blue).
I guess maybe the difference is brightness and the brighter star has more capability to saturate your colour receptors.
Take a look again -- see what you think.
Best,
Les D
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