ultra... as in ultramel?

Yes, there is no reason that you cannot create an ultramel-pinstripe-pastel, and the way you suggested would have a shot at accomplishing it.

Keep in mind:

lemon blast x ultramel = 1/4 chance per egg of a lemonblast het ultramel

lemonblast het ultramel x ultramel = 1/8 chance per egg of hitting on lemonblast ultramel

You may need more than a handful of seasons if you're unlucky.