Shopify Vintage Themes (Not 2.0 Themes)
From Ongoing --> Click Settings
From Settings --> Click Subscription Widget
Here you can turn on or turn off the BUY NOW button & also customize the colors
Shopify 2.0 Themes
The BUY NOW button will be controlled by your theme, and not Ongoing.
To make adjustments, you can go to Shopify Dashboard --> Online Store --> Live Theme --> Click Customize --> and you can use the theme builder to adjust the appearance of your product pages.