Here is the formula you want.
P^n > 0.99
P = probability of a single egg not producing a pastel
^ = to the power
n = number of eggs.
The probability of producing a pastel is 25% = 0.25. So P = 0.75.
By a little trial and error with my calculator, n = 17. So you hatch 17 eggs. Every baby is normal, enchi, bright enchi or something other than a pastel. Then you have a 99.2% probability that the bright enchi parent is not a pastel enchi. More eggs raises the probability, but 99% is a good cutoff point.
If one or more pastels hatch, then you can stop before 17 eggs, of course.
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