After taking a peak at your account, it seems that your email editor is set to Basic editing. The easiest way to make these changes is if you switch that to Advanced editing. This will give you options to change the colors throughout your email!
On the top right, you should see this button:
When you toggle this button over to Advanced Edit, you will then see a list of options for global Colors & Fonts on your left:
You will then be able to click on the color you want to change and select a different one from the color palette:
I hope this helps, let me know if you have any other questions.
Apologies for the confusion. Your account is already in the advanced setting. You should see all the color & block options on the left while in your emails.
Hope that helps!