Cortana is available for Windows Phone 8.1, Windows 10, and Windows 10 Mobile. On Windows platforms, the colour of Cortana depends on your device colour.
Cortana was previously available for Android, Invoke (Smart Speakers), iOS and the Xbox One games console. However, Cortana never caught on as a personal digital assistant and in 2020 Microsoft is reorientating Cortana to be based more on Office 365 assistance, rather than being a personal assistant like Alexa, the Google Assistant, and Siri. Part of this process will involve Cortana being separated from Windows 10's core code and packaged into a PWA app with a fluent design UI, this will be distributed and maintained (updated) via the Microsoft Store.