Hi evoluke,
As long as the purple colored plants are healthy looking, don't worry. The dominant leaf color in snapdragons is green. However, some plants may carry a recessive gene for purple leaf color. In your case, some of your seed had received the recessive gene from each of its parents thus producing plants with purple leaves. Enjoy them!
BTG

I really doubt anything hereditary/genomic is at work in the case of the purple leaves. ;o) An over-accumulation of free sugars will stimulate anthocyanin (purple pigment) production, which can cause leaves or stems to take on a reddish or purple color.

The most common cause is a deficiency of phosphorus (P) because P is required to make ATP and ATP is needed to turn sugars to starch and to load sugar into phloem for transport. No P - no ATP - no move sugars/starches - anthocyanin builds up - plant turns purple. The most frequent cause of a P deficiency is cold soils, which is why so many plants planted out too early turn purple.

A P deficiency is not the only suspect cause of purple leaves. Nearly any environmental condition that puts the brakes on growth and the accompanying use of sugars, but does not limit sugar production (photosynthesis) can cause anthocyanin buildup and purple leaves.

If only the outer edges of the leaves are purple, it may be a K or Mg deficiency. If the center of the leaves are also purple, it could be too much Ca in the soil or irrigation water blocking uptake of P and Mg.

Al

In my experience, when you grow snapdragon 'mix' seeds, the purple coloured plants are going to bloom a very deep red. Assuming plants are otherwise healthy (and in my experience, these purple plants are usually very strong seedlings), there's no need to worry.

There are several snapdragon seed strains that have bronze/purple foliage. I think the flowers can be any colour.