UI Design Frameworks

Two other options you can consider are APIs and WebContent.

You can possibly use your front-end as is, and configure it to call Appian Web APIs to start processes.  This gets you a way to use Appian exclusively for your backend.  This is an approach, but possibly a poor one, because you miss out on a lot that Appian has to offer.  The wiring between the two is going to be complicated, and using the CSS and design framework is likely going to be complicated too.  You're not going to be able to build your app as quickly or maintain it as easily as you would if you had just used SAIL.  You will need developers that know both Appian and the other technology.

There's also a web content component that lets you put basically anything hosted on the internet into a pane that exists within the Appian SAIL interface.  A common use case for that is a Goggle Map. It's possible that you could leverage that to build your whole interface in another language and then display it from inside SAIL, but that approach might come with even more of the problems listed above.

For colors in particular, many components such as rich text, box layout, card layout, and others let you configure colors on the fly to any valid HEX value.  You can also set font site-wide for sites, and set default colors for all components site-wide for each site.  You can change a fair amount of styling for buttons and other things like that as well. Appian is relatively flexible, especially when it comes to colors.  And it keeps adding more branding flexibility almost every release.

Two other options you can consider are APIs and WebContent.

You can possibly use your front-end as is, and configure it to call Appian Web APIs to start processes.  This gets you a way to use Appian exclusively for your backend.  This is an approach, but possibly a poor one, because you miss out on a lot that Appian has to offer.  The wiring between the two is going to be complicated, and using the CSS and design framework is likely going to be complicated too.  You're not going to be able to build your app as quickly or maintain it as easily as you would if you had just used SAIL.  You will need developers that know both Appian and the other technology.

There's also a web content component that lets you put basically anything hosted on the internet into a pane that exists within the Appian SAIL interface.  A common use case for that is a Goggle Map. It's possible that you could leverage that to build your whole interface in another language and then display it from inside SAIL, but that approach might come with even more of the problems listed above.

For colors in particular, many components such as rich text, box layout, card layout, and others let you configure colors on the fly to any valid HEX value.  You can also set font site-wide for sites, and set default colors for all components site-wide for each site.  You can change a fair amount of styling for buttons and other things like that as well. Appian is relatively flexible, especially when it comes to colors.  And it keeps adding more branding flexibility almost every release.