I try's the sample applications and you are right that all is working.

The problem is that the sample applications are not setting any params. As soon as you add my param list the output colors are also washed out.

I am using the following params to speed up decoding of x-trans files (For Canon/Nikon it doesn't matter). Without those parameters the encoding is much slower. But if I set only one parameter the color are washed out independently which parameter I set:
use_camera_wb$set(params$slice, 0);
use_auto_wb$set(params$slice, 0);
output_tiff$set(params$slice, 0);
half_size$set(params$slice, 0);
user_qual$set(params$slice, 0);