As I understand your question, you'd like to change the red callouts to blue, correct?
I gather you have many, many slides already. If that's the case, pick one of your callouts and change the color to your desired version of blue.
Then select the callout you just changed (if it isn't already) and double-click the format painter icon found under the Home tab.
You would then go through each slide and click each callout once. This would change the color instantly. Beware that your callout may have different states. If that's the case you'd need to adjust all states to your desired color before changing it.
Hope this helps :-)