I'm not realy sure what you are asking?
Could you give an example of your hsv and equivalent rgb values.
Basically the Color.from_hsv expects the values to be in the range of 0 - 1.
The saturation and the value are already in the 0 to 1 range.
This leaves the hue which is between 0 and 360.
To have it in the range of 0 - 1 you need to divide it by 360.
In the sample you put up there
var c = Color.from_hsv(210/360.0, 0.5, 0.79, 0.8)
Is this what you are looking for?