Luckily we've got a fair-sized garden, so she's able to get out safely. She doesn't show any inclination to go further at present.
No she isn't white - got some black as well, but with a white head - and her eyes aren't blue. We'll get her ears checked in case it's just wax, but we don't think it is.
One thing we've noticed is that she's very cautious about pushing open a door to go into a room - more likely to sit there till it gets opened for her.
Gradually she's getting to be less suspicious of our other cat, who's been very patient in accepting that she isn't too friendly to him. He's used to having cats who treat him with affection, so it must be a bit confusing for him. But now she's become willing to eat alongside him, with individual plates.