If you are just starting here because of a google search, I would suggest going back and reading all the entries labeled
Pastel Hair by clicking here
http://bobbypinblog.blogspot.com/search/label/Pastel%20Hair.
This is only one in a series of blog entries about experimenting with a
vintage pastel hair color looks and there may be something useful in
another one of the entries or comments that you need to know before you
begin.
I am writing first about the most frustrating color I dealt with...pale
blue. This color is great if you are starting with pale grey hair or
cool white blonde. Blue (ash) is temperamental. It is very common for light
hair to turn green when you put blue on it or if the hair is yellow
blonde, when the yellow and blue mix together you get a green-blue. Part of the problem is I have a very specific color idea in my head and so it is hard when you don't reach that color. I am not going for deep punk rock blue. I want something you would see at Easter.
No this isn't really Katie's hair color. I photoshopped it to what I
would love to see her wear, even in a wig. It looks great with her eyes.
I know we are not all so lucky to be able to get our hair to a cool blonde. If you have black coarse hair, have died your hair jet black, or over processed with color changes, you can turn your hair to cotton candy trying to get it light and cool enough to put a pastel blue over it. If you can't get your hair a cool white blonde and still have some yellow or peach in it, pale lavender or pink will be much easier to achieve. If you can get your hair to a cool white, go for it. I think pale blue is beautiful and fresh. Then you can truly be a blue-haired old lady. :)
I actually got a pretty cool mint green, but that isn't what I wanted. I
want a true pale blue hair color, not pale teal or mint.
These tests were all done on pale yellow/peach hair swatches so you can see how the blue mixes. I got a couple swatches to turn out pretty nice, but they would be even nicer on a cool blonde or grey. Interestingly, I had the most luck with inexpensive demi-permanent hair color in these tests. I used a lot of the
shampoo cap method on these, so please see my post previously on how to do this.
Here are some decent successes. I will include brand (with links), color, and processing time in all of these. Some of these brands are only available to professionals, so you may have to hit up your beautician friends to help you.
See Shampoo Cap details here.
http://bobbypinblog.blogspot.com/2012/05/pastel-hair-color-shampoo-cap.html
This swatch used
Crazy Color in Sky Blue.
Crazy Color
is a demi-permanent UK brand and I didn't find a US supplier, but you
can order it online. It was applied directly and only left on for 3 minutes. This could get a little tricky. The hair sucked up the color really fast. Make sure your hair is in good shape or seal it with a toner to even out the porosity. I like the color a lot, but you would have to do your hair in sections as a direct application so you could get it on fast and remove it fast. By the time you would apply the whole head, your time would be up. That is the beauty of the shampoo cap for these fast applications. I am literally trying to just stain the color.
I retried this using Sarah's suggestion in the comments of a
shampoo cap only replace the shampoo with conditioner and left it on for 15 minutes. I got a really nice blue like this with more application time. It was a little dark, but blue will fade. If you are using equal parts conditioner and color, you should be able to smear it in your hair quickly and evenly with gloves.

Test using
Crazy Color Capri Blue as a
shampoo cap processing for 20 minutes. This one came out pretty good. It is silvery-blue
with only a hint of mint in it. The mint is where the yellow blonde is showing through a little, so that could have been avoided. I have no doubt that on a cool blonde or
grey, this color would have been just right. But I like how nice it looks at such a light shade. A lot of the colors I tried looked really dingy at as a light shade.
This was so far away from what I wanted, but I still kind of like the way it turned out. This was a
Pravana hair color mixture. I combined equal parts of
Vivid Blue and Green with 10 volume and applied it for 15 minutes as a
shampoo cap. It turned this mint green...kinda cool. Not green in a Halloween kind of way. It was a very '50s looking mint.
This one was a little darker than I was going for, but I really love the cool blue of it. It is
Beyond Zone Color Jam in Huckleberry Blue applied as a
shampoo cap with a 15 minute processing time. I think this color would fade really nice too. It counteracted the warm blond really well without turning green. If you are working with super light white hair you could cut the processing time 5 minutes and get a beautiful shade.
This one almost turned out grey. I like it. Not what I was going for, but I like it. This is 1 part
Joico K-Pak Silver Blonde, 1 part
K-Pak Royal Blue Intensifier, 40 volume and 30 minute processing. I wonder how it would look if I had added a little purple intensifier to it?
Most of the blues I used were too dark, turned teal/blue or the deposit looked dingy and irregular. The
Crazy Color blues went on the most smooth and looked the nicest when light.
The Manic Panic blue I tried is great for vivid colors, but did not work well with a shampoo cap. It couldn't penetrate the hair shaft very well with the barrier. I saw an image on Manic Panic's website from a photoshoot that a Orlando Pita worked on using Manic Panic colors and they achieved a pretty pale blue with Manic Panic. I used a few other color companies, but had the same issue with either too dark or didn't look good in a light form.
Too dark. This was a test using
Chi Chroma Shine demi-permanent color in blue applied directly over the hair and washed out after 10 minutes. Their blue was pretty, but looked best dark. As a shampoo cap the color wasn't saturated enough and looked dull.
Too dark. This is the same
Beyond Zone color Hucklberry Blue I liked up at the top, but applied directly instead of the shampoo cap and washed off after 5 minutes. You can see what a difference the shampoo cap can make. The color is uneven. I could have gotten it more even if I had taken more time to apply, but if I left it on too long it would be too dark, a double edged sword.
Too dark. This was
Pravana Vivid in Blue with a shampoo cap application. These
Vivids are great if you want vibrant color. I use their red hair color on myself. They are really strong, but it was too strong for what I wanted. I was surprised that even with the shampoo that this came out so deep.