The London "It" girl's '90s style wouldn't be complete without a complete hair color change-up every so often.

Try this: For Ora's blue and purple pony, brush on Bleach London Hair Crayons in Washed Up Mermaid and Bruised Violet, $6.71, or create your own color combo.

For a quick pop of color, Kahlani recommends using hair chalk on braids. "Plait the hair, then put the chalk on top of a few sections and set with hairspray," she says.

Who it works on: Light blondes, ashy blondes and golden brunettes can easily copy Ora's deep blue and purple look. But if your hair is darker, you can still get the dip-dye effect, explains Kahlani, by going for deeper reds and pinks like Bleach London's I Saw Red and The Big Pink.

Yes, her lavender shade was pastel perfection, but her current dark blue is much subtler -- ideal if you're thinking about transitioning colored hair into your everyday look. Plus, you can tint the hair without having to bleach your whole head first.

Try this: Manic Panic Semi-Permanent Hair Color in After Midnight, $13.99, is our go-to match for Richie's look, but it's designed to last several weeks. "You can use dishwashing soap if you've got to get it out quickly for work," Kahlani says of getting the dye to fade faster if you find yourself in a pinch. "Cleansing shampoos and just washing it a lot will help, too."

Who it works on: Lighter hair colors will get a more vibrant blue, whereas dark hair will get a dark tint.

"I'm in the process of going through the entire rainbow," Kesha said in a recent interview. And true to her word, she's since been spotted with shades of lavender and baby blue this summer.

Try this: To get Kesha's light pink shade you don't have to worry about coating every strand perfectly -- it's more of a highlighted look. Divide the hair into sections as you roughly spray on a pale color like Bumble and Bumble Spray Chalk in Blush, $19, throughout the hair.

Who it works on: If you have your heart set on the cotton candy pink or light lavender shades, and your hair is naturally dark, you'll have to go to the salon first to lighten your hair. The process of going that light can be ultra-damaging, so don't try to do it at home, Kahlani cautions.

But if going blond for the sake of pink strands is not on your wish list, try picking up some stencils from your local craft store. "Those show up great on dark hair," says Kahlani.

Hudgens posted this pic on Instagram when she turned her signature bleached ombré hair into a cool aquatic color.

Try this: Rita Hazan Pop Color in Blue, $18, is an easy-to-use spray-in color that washes right out. "What's quite good is to get the hair smooth and then spray it at a little bit of a distance, working downwards," Kahlani says of getting the look. "[Spray it] heavier on the ends so you get a more intense color."

If you don't naturally have a wave to your hair, but want to copy Hudgens' curls, Kahlani recommends heat styling before you add color. "Heat removes the color, so do all your prep before sprays and chalks," she says.

Who it works on: Light and ashy blondes and dark hair with bleached ombré can get the pale blue shade when sprayed on lightly, while dark haired girls will have to spray the product on more heavily to get a turquoise shade.

No stranger to experimenting with electric hues, Lovato's latest shade of purple is a fun pop of color that's surprisingly easy to get at home.

Try this: L'Oréal Professionnel Hairchalk in First Date Violet, $24, is applied like a dye on dried and styled hair, and will last through several shampoos. Apply the color to your hair in sections -- remember, the more heavily you saturate the hair, the longer it will last. After each section, allow the color to dry for one minute, then brush through while lightly blow-drying.

Who it works on: The pearlescent pigment technology means L'Oréal Professionnel Hairchalk shows up on every hair color. The downside is, unless you employ Kahlani's dish soap tip, the color is designed to last for four to five shampoos.