I think you are 100% correct.
I would think the strange colours that appear in sunlight might be something to do with the refraction of the light.
If you look at a piece which has the frothy yellow background with blue trails on it (as in the design with the three spirals on it), with the light coming through it, it will show all sorts of muddy purples and browns. But there is only yellow and blue glass there.
When light comes
through coloured glass, it will reflect the colour on the other side - only allowing the wavelengths which are not reflected to come through the glass, so those are what you see.
Colour in glass does not work like paint.