I see. Well I think that's quite difficult to do in WatchMaker. The watch face should be designed for the ambient mode instead of let the watch filter the colours. Watchmaker is only showing preview of the ambient mode how it was designed, which is I think good.
refering to https://developer.android.com/design/wear/watchfaces.html#SpecialScreens
Ambient mode
"The background color scheme is strictly limited to black, white, and grays."
"but" in Optimize for Special Screens
Reduced color space
"In low-bit mode, the available colors are limited to black, white, blue, red, magenta, green, cyan, and yellow.
Other displays save power in ambient mode by not producing any color. When designing for displays which do not use color in ambient mode, the background may be either black or white.
"
seriously lots of watch face looks weird without trying on the watch itself.
a warning message would be nice i think. I remember java could read color, when it detected color that is not for the watch it will warn?
I absolutely agree with you that most of the watches look really bad in ambient mode. Mainly because they are not designed according to the recommendations (low-bit colors or black& white only, shapes in contours only to prevent the burn-in). So the problem is in the people designing the watch face, not in the WatchMaker.
WatchMaker Feature Requests
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