I have sent the seller the following:
For information: All of the dishes like this one that I have checked with uv light show that they were made in the later Vasart Ltd period or perhaps even later in the Strathearn years. Therefore, they could not have been made by Salvador Ysart, who died in 1955, and Paul Ysart never worked at Vasart.
I did not comment on the point about the ground colour as that would be too complicated to get into in an eBay message. It is possibly true that many of the colours used by Salvador in the Ysart Bros (Vasart) period tended to be less bright than those used by Paul in the same years - but there are also many exceptions to that idea. However, items with greater glass clarity, which can reveal the colours as much brighter (or lighter) are more likely to be made of the lead-based glass of the later Vasart Ltd or Strahearn years. And my uv checks tend to confirm this - so on that basis Salvador never made the item and Paul, in any event, had nothing to do with the Vasart production.
Nevertheles, it's a nice example of a paperweight-based dish.