This info was taken from Jeff Ronnes site he is the man that named the first Pastel boa.
A Pastel is this: A Boa that has an odd overall wash lacking the normal amount of black and a reduction in black pigmentation in particular throughout the pattern. This is particularly apparent in babies, which have the same kind of washed out pattern as Hypos. The saddles as well as the side blotches have less black than normal In fact, often the side blotches have no black whatsoever. That's it, no more no less. Now it is unfortunate that this leaves a lot to interpretation. Identifying Pastels can be and is subjective. It is very much a matter of opinion. There are definitely degrees of Pastelism. How much Pastelism is required to label an animal Pastel? I don't know what measure others may use, but I know what I look for in babies. I look for nearly no black in the side pattern. A very small amount of black can be found but nearly none normally.
The History of Pastel Dream Colombian Boas. - The Boa Network Forum Boards
A pastel in my opinion has brown / tan saddles with no black outline in them and the side medallions have very little to no black in them. Also a pastel does not have to be pink or red to be a pastel they can be gray sometimes only reduced black as Jeff describes. With the reduction of black in them the lighter colors such as pink and red stand out more.
Here are some pictures of my Pastels I have owned.
You can see the color difference in the normal boa and the pastel boa in these next two pictures.
The pastel boa is the bottom one in this picture and the boa on top is just a normal boa with a lot of red in him.
Take care
Tom