Reading Color(0xAARRGGBB)
The 0x just means "this is hex". After it come four pairs: alpha, red, green, blue. In Color(0xFF3A7BD5), FF is fully opaque, 3A is red, 7B green and D5 blue.
Alpha goes from 00 to FF. Half transparent is about 80, so Color(0x803A7BD5) is the same blue at roughly 50%.
Where did my colour go?
If you copy #3A7BD5 from a design and write Color(0x3A7BD5), Flutter reads the missing pair as alpha 00. Your colour is there, it's just completely transparent. Always put FF on the front of a six-digit hex code.
It's also alpha first, unlike CSS, which puts it last. An 8-digit hex from a web project like #3A7BD580 needs its last pair moving to the front: Color(0x803A7BD5).
Other ways to make one
If hex feels fiddly there are constructors that take plain numbers. Color.fromARGB(255, 58, 123, 213) takes alpha and the three channels from 0 to 255. Color.fromRGBO(58, 123, 213, 1.0) takes the channels, then opacity from 0 to 1, much like CSS rgba().
Stick const in front when the value never changes, const Color(0xFF3A7BD5), and Flutter can build it once at compile time.
Flutter, Android and the web
Flutter's layout is the same as Android's, alpha then red, green, blue, so values copy between the two without any shuffling. Just swap # for 0x. The web is the odd one out, with alpha at the end.
If your designer hands you colours as rgb() or HSL, paste them into the box above and copy the Flutter line. Saves a lot of squinting at hex.