These are from a pack of mixed seed? I would assume that the dark purple just means that these plants will be have very dark flowers. A nutritional deficiency would more likely show up as pale leaves of yellowing leaves, not purple. What kind of soil did you start them in?

Tomatoes make some of the same pigments of the red-purple variety too, though in their case you will not see much of it until the fruits turn red. In many plants the brighter green of the leaves masks the red family of pigments. They are there, but we can't see them...until fall, for many plants.

I have several snaps with some leaves that are purple, depending upon the colour of the blosssom. No problems here; my snapdragons are very robust.

Purple leaf discoloration could reflect a phosphate deficiency - or soil temps too cool for plants to take up this nutrient.

If growing conditions (light, temperature, ample root room) are OK, it's doubtful that you'd want to apply any more than a weakly diluted balanced fertilizer application to seedlings.

Eric is right about the phosphorus and purple leaves. It's very common on many plants in cool soil (seedling or mature). If that's is what going on with you don't worry, as soon as the soil warms they'll regain green :D
However I have Snapdragon 'Black Prince' who do have a purplish cast to the leaves in summer. Right now in the cool soil all the new growth is VERY purple. Here pics of one of them a few days back. BTW these have been perennial for me. This here particular one never completely died back either so it was purple all winter also...
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My darker flowring Sweet William (vs the light colored) also have deep purple to maroon colored foliage when it cools in fall and again in spring...they green up before they flower. I assume since the white and pink flowering ones dont turn purple it must be indicative of bloom color :D
Also I noticed purple Echinacea has dark maroon early spring leaves vs. the White Swan which is bright green!
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Vera