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This is normal and you can encourage more blossoms through deadheading. This article will help you with deadheading:
https://www.gardeningknowhow.com/ornamental/flowers/fgen/deadheading-flowers.htm

Snaps usually have a few bottom leaves looking pretty bad. If the rest of the leaves on the plants look OK, it might just be a normal thing. Has the soil been too wet, they hate that! You might want to take a sample to your local agricultural extension office for identification and recommendations.

You should plan on planting them annually. But they are actually a tender perennial. Most of the time, they will die off, but once in awhile they end up in a sheltered location and will regrow the next year. But they do not do so consistently enough to rely on them coming back every year.

You are likely seeing the offspring of the plants or there was a few plants that were mislabled. While they did not have a cross pollinator, they likely are hybrids and their offspring will not look like them (this is done intentionally by the seller to prevent people from collecting seeds and growing them again next year). And mislabled plants do occasionally occur. Whatever the cause, the flowers will not go back to yellow.

I use some Epsom Salts on our Snap Dragons once a year and they seem to have sturdier main stems due to its use. Some gardeners say to have your soils tested first, as some soils are already high in magnesium and adding the Epsom Salts may not really be needed. No need adding something if it is already there in abundance and adding to possible soils salts issues. Here is an article for you to check out on the use of Epsom Salts: https://www.gardeningknowhow.com/garden-how-to/soil-fertilizers/epsom-salt-gardening.htm

Here is an article for you as well on Snap Dragon growing and care: https://www.gardeningknowhow.com/ornamental/flowers/snapdragon/growing-snapdragon.htm

Enjoy those talking blooms! :-)

For directions on how to plant, please visit the following link:
http://www.gardening.cornell.edu/homegardening/scene95d7.html

For more information on how to care for snap dragons, please visit the following link:
https://www.gardeningknowhow.com/ornamental/flowers/snapdragon/growing-snapdragon.htm