Form 'Button Colour' allows you select from the Primary, Secondary and Accent Colours that you set up for your brand in the 'Styles' area.
USE CONTRASTING COLOURS
You have many colours to choose from in your colour palette, found in the 'Styles' area, so always pick a colour that has the most contrast allowing it to jump off the page.
Your Forms on MOBLE sit in a frame allowing you to change the Background Colour of you forms in moments. Therefore, it can be a good idea to create form variations with different colours allowing you to select the right form for the right instance.