Look for the most blushed out head that you can and the least amount of orange for a pastel that is likely to hold its color the best into adulthood.
Lemons do tend to go through an ugly duckling stage before they turn into swans though.
Here's my lemon pastel male at 3 years old.
And here's a progression of his babies:
See the super blushed head?
After first shed:
A little later:
Even later:
Color starting to come in:
And for the most updated pictures of her, go to my website (linked in my signature) and go to The Nest and you can see what she looks like today.
What remains consistent in her pictures is the super duper blushed head. When I look at pastels at shows, I pass over the ones with heads with little to no blushing. The uber faded heads are the ones that will be show stoppers as adults.