I don’t think the change in the coloration of your Sunburst is anything to be concerned about at all. Sunbursts tend to exhibit the sunset colors when conditions are favorable, and these include various shades of gold, yellow, peach and orange (including pumpkin orange) so the pumpkin color it has adopted now is nothing out of the ordinary.
In fact, like other seahorses, you can expect Sunbursts to go through quite a range of color phases from month to month over the long run. If the pumpkin color your Sunburst has assumed is quite close to the color of the rockwork and decor in your aquarium, it may well have shifted its coloration to better blend into its background. If the Sunburst had been maintaining yellow in coloration while the yellow coral was in the tank, and has now adopted the orangish coloration instead, it’s certainly possible that removing the yellow coral may have had something to do with the wardrobe change your seahorse has undergone.
But it may just as well have adjusted its background coloration for another reason altogether, as seahorses are wont to do. It is difficult to determine why and when these chameleons of the sea prefer one ensemble over another at any particular time.
But I certainly don’t think the change in your Sunburst’s coloration indicates any sort of a health problem, Haynes. The circumstances in which a change in coloration could be problematic are when there are white blotchy patches or a localized loss of coloration (depigmentation), which can be a sign of a skin infection, or when a seahorse assumes an unnatural pallor all over its body, which can be a symptom of hypoxia or low oxygen levels, or when a bright seahorse darkens in coloration, which can be a response to stress.
In short, a Sunburst changing from a shade of yellow to a shade of orange is perfectly normal and not an indication of trouble, Haynes.
Best of luck with your new Sunbursts, sir!