Hi Javier,
Yes, I suspect that it was flashed lightly or had a small amount of fine coloured enamel marvered in. The result being a gradation from edge to centre - it looks very well done as it gives the visual effect of the glass thickening to the edge much more than it does. If it was not for the small streak of colour at the centre, I would still be scratching my head about how it was done.