Yes! You can enter a hex code directly into all color fields (also applies for Campaign colors). On a PC, the color picker allows you to enter specific colors using by Hue/Sat/Lum or RGB values. On a Mac, the color picker looks different, and there is no option to enter Hue/Sat/Lum or RGB values.
In the Theme section of the Landing Page creation, you may customize the colors of buttons and links with the 'Primary Brand Color', select a Page Background color, and upload additional imagery for the background of the page and a banner.