add cyan color to drop down menu for LUT (fix #498)

[Rdornier]

Adding the choice for Cyan color in the display LUT.
Fixing #498 issue

[will-moore]

Looks good!

[Rdornier]

I just see now that I could also add the cyan color to scalebar / label drop down menus (to be consistent).

[jburel]

Should more colors options also be added to the scale bar color menu for consistency?

[will-moore]

@jburel I think it's a bit more work to add the "More Colors..." option to scalebar, as you need to create and handle the color-picker response.
But @Rdornier might want to add the Cyan option to scalebar and label colour menus?

[jburel]

@will-moore Cyan has already been added and it works
so let's keep it like this

[will-moore]

Sorry, yes I missed that (was looking after the comment at #516 (comment)). So I think this is good to merge then 👍