The Pastel name came from Jeff Ronne. So, I will use his description of a pastel.
It is an overall reduction of black on the boa. Color has never been a "pastel trait".....But, a by product. Ronne has line bred this animals over time to get this effect.
A good example of a pastel line that is not pink are Davey Pastels. They are really yellow looking with the overall reduction of black. Great looking animals if I do say so myself.
Ronne's Lucy bloodline is REALLY pink. But, the color does not a pastel make. 
I just had a litter or pastels. 90% of them are pink in color. But, there are varying degrees of pastels in the bunch.
So, a reduction of overall black makes a pastel. Not the color.
Co-Dom or not? Some claim it. Ronne has yet to. Personally, since he has coined this name and description....I feel it is up to him to say it is....Or, not.
Then there are Harli boas, they have been proven to be co-dominant in nature.
What I know from my small breeding experience is this.....
I bred my anery pastel male with my Ronne Pastel female....Out of 21 babies....I would only consider 1 to not be a pastel. (Technically he is..But, I'm pretty picky when it comes to what is and is not a pastel.)
Now, isn't that what happens when you bred a hypo to a hypo? You'll get maybe two normals and the rest are hypos.
So, to me...It "acts" co-dominant in nature. But, I don't have a vast number of animals to prove this out correctly. (Hopefully someday)
Buyer beware. There are a TON of people saying animals are pastels and they are NOT!! They are just nice looking boas.
This is my best pastel out of the bunch!

This one is the most colorful on so far. But, remember color does not the pastel make. 

Hope this helps.
John
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"To be the best..........You must lose your mind."