Answer: There is nothing that will do this for your begonias. Variety selection is the best way to get another shade of flower color.
Sometimes temperatures and sun exposure will affect flower color, and leaf color, in begonias. Flower colors seem to be most vibrant in cool weather (where can that be found in south central Texas during the summer!?). You might try moving a few to a brighter or shadier location to see if it is a significant difference. More sun should make the foliage darker on varieties with reddish tinted foliage, but sun may wash out the color on the blooms a bit.
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