Such a great questions and us horticulturists have been fascinated by the flower color change  since ancient times. In general it is weather, the pH or acidity of the soil and the type of plant that all contribute to the change in petal or flower color. In your case and at this time of year (early spring) I am going to guess that the cool nights have contributed most to the color change of celosia not the fertilizer. If you were asking about a hydrangea I would say the fertilizer or soil additive would be the one thing most responsible for a change in color. Then some plants (many roses, pulmonaria, sedum Autumn Joy ) just change color as part of the maturing process of the flower. When if comes to those more vibrant poppies you have it could be the iron in the Osmocote that have allowed the vigor of the plant to put more punch into the petal color. Healthy plants have better color. Gardening is a fascinating way to observe all the remarkable changes in the natural world and you can also experiment on your own and see if you can keep some of your celosia plants covered at night or closer to the house to test if temperature is changing the color. Celosia likes heat so cool nights can cause a bit of trauma. Keep growing, Marianne Binetti