Stephanie here from Shopify.
Typically you can modify colors for buttons within your theme's general settings (Online store > Themes > Customize theme > General Settings > Colors).
It will depend on your theme though if this can be modified through the theme editor or if it will require a change within the CSS file of your theme. Would you be able to post a link to your store? That would be super helpful!
Feel free to reach out to our support (http://bit.ly/2uVU8jk) if you have any other questions! We're available 24/7.
The Symmetry theme will have an option to change the 'sold out' button within the theme editor within the colors section (Online store > Themes > Customize theme > General Settings > Colors)
The "Out of Stock" button color will have to be customized within the code. I'm afraid I'm not the best person to assist with that as I'm not a developer; however, perhaps someone else can hop in with those instructions here!
Alternatively, you could reach out to your theme's support to see if they can point you in the right direction to make that CSS change!