What to wear to a TOPSHOP interview

I even get sick of these threads so sorry if it's been posted a million times before.

I usually just wear a black dress and tights or a black top and trousers, however I'm worried I'll look too serious and like I'm going to a funeral.

I have a resting ***** face, and I want to look as happy as possible and I don't think wearing black really helps, which is a shame because I do look great in black.

So what do you think and are jeans definitely too casual for an interview?