i think you will be sorry if you use any color, semi permanent or not. my daughter had semi permanent put into her hair to cover gray, was supposed to wash out in a few weeks... it's almost 3 years, and the last few inches of her hair are an orange...
try experimenting with styles. i saw one, i think on goinggraylookinggray.com once that i thought was so cute. she had very narrow braids just in the front part of her head, and then her hair, grey, fell into the back, black, but the style was soooo very pretty. if i can find it, i'll post it here.
for one night, it is not worth setting yourself all the way back. and, lots of people are letting their grey come in now, so it's kind of 'in.' honestly, i thought my hair look weird, scary, but i had many, many people friends, and STRANGERS, walking up to me to tell me how much they liked it!
and, confidence is really all you need!