The button color in email sequences is conrolled by the color within your brand.
If no brand color is set or you would like to change the brand color here are the steps to change your brand color.
Click on Account Settings ---- Select StickyEmail ---- Set Brand just below your contact details.
Note: Call to action buttons will work in all emails sequences. Color is supported only in sequences that support brands. This would be any sequence created after June 30th, 2018. Sequences created prior to this date would default to dark grey.