Hi Catalina, It looks like you're using a template for dynamic-remarketing. In that case those blue colors are pulled dynamically from the feed. When you run the ads those colors will be set in the Adwords UI. For testing on your desktop, you can change those colors by editing the file "five_product_feed.json" in the "feeds" folder of your GWD layout. The file may be hard to read as is, but if you format for JSON you'll see several "Design_0_" parameters and I think the one you're looking for is "Design_0_btnRollColor": "#005cb8", You could change that to "Design_0_btnRollColor": "Green", for example.
Another approach, if you don't need dynamic colors, is to disable them. In GWD, select the item, open the Dynamic panel, and delete the color binding. For the CTA, it looks like: "div #cta-off > Background color >Design.btnColor | hex" click the x next to that rule in the Dynamic Properties panel and then it will respect the colors you assign in GWD.
Hope this helps,
Google Web Designer team