Is there a way to force the ambient mode that does not reduce the color depth?
I think not. It would help if we have an idea what's happening when an Android Watch goes to Ambient mode.
Ambient
mode
helps the device conserve power. Your design should make clear to the
user that the screen is in ambient mode. The background color scheme
is strictly limited to black, white, and grays. Your watch face may
use some color elements on screens that support it provided it is
unambiguous that the device is in ambient mode. You can use color
elements for up to 5 percent of total pixels. In this mode, the screen
is only updated once every minute. Only show hours and minutes in
ambient mode; do not show seconds. Your watch face is notified when
the device switches to ambient mode, and you should thoughtfully
design for it.
So, your watch losing the vivid colors in Ambient mode is it's way of conserving battery power.
You can also read more about Ambient mode here.