I KNOW it's not the same but, at a pinch, can I use Yellow for Black in 'Gujerati Beans'??

If you can't get black (or even brown) mustard seeds you could be better off using Nigella Seeds (aka kalonji or Black Onion Seeds) for texture & appearance together with some gratered horseradish or wasabi for potency rather than using the yellow seeds.

yes

Hi, I think Lapis was saying 'yes' to your original question rather than to my suggestion. You can use yellow mustard seeds of course, it will just be very different!

I was saying yes to using any mustard seed, I don't think it makes much difference to the final product, its just what is available in which geographic location.