Pastel Yellow Experience? Roux Fanciful?

I've only ever used full strength yellow for rainbows. I want a soft yellow glow this time. I'm wondering whether I should put it on full strength and fade it, or if I should dilute it like the pinks and blues that I'm familiar with. I'm thinking closer to full strength to prevent it from getting mucky the first rinse, but my ocean swimming ways will fade the other colors to nothing the first go.

Am I just trying to convince myself to do a full-on rainbow before I cover it all up for the audition season?

Which brings me to my second question: Will Roux Fanciful blondes cover pastels enough for me to do an audition at a company I've never worked with before? Prior to this, I had worked with established theater groups who knew me and were willing to wig me in exchange for my talents. I'm on my boyfriend's comp, so I don't have pictures handy, but I'm an ashy pale blonde with overtones of pastel blue and pink, several inches of medium blonde roots, and an inch of my natural brown. I haven't been dark in years, and the prospect is terrifying!