Which is better in terms of staying power: SFX Electric Blue, or Blue Mayhem? And how often did you wash your hair while it was coloured with the above products?

Electric Blue lasted a bit longer than Blue Mayhem on my hair. Electric Blue tends to turn kind of indigo after a couple weeks, since it's purple based, but it lasted as indigo/violet for a couple weeks after that. Blue Mayhem, on the other hand, turns pastel blue after two weeks, then silver after another week.

Whenever I've had them in my hair, I was washing my hair once every other or every three days with a gentle, conditioning shampoo.

Blue velvet it unbeatable for longevity. Just depends if that's the blue you're after though because it's quite purply.

I agree, Blue Velvet lasts forever, Electric Blue a few months (but does get a bit purply, I used to make it bluer by adding some turquoise), Blue Mayhem lasts a bit less than electric blue but is a truer blue shade.

I once used blue velvet and mixed it with burgundy wine, it came out like the color of a blue pen with the reddish highlight in the sun. but anyway,even though I mixed it with the burgundy color it still lasted over a month with unbleached dirty-blonde hair.

Blue velvet is a little darker than what I'm looking for and it looked greyish after a few weeks in my hair.

I think I'll go for Electric Blue and since I'm getting turquoise anyway, add a bit of that if it's too purpley.

Thanks guys!