The imagick PHP extension is irrelevant. Kirby supports ImageMagick if you enable it via the thumbs.driver config option. Otherwise gd is used.
This is what google says about webp color spaces
What color spaces does the WebP format support?
Consistent with the VP8 bitstream, lossy WebP works exclusively with an 8-bit Y’CbCr 4:2:0 (often called YUV420) image format. Please refer to Section 2, “Format Overview” of RFC 6386, VP8 Data Format and Decoding Guide for more detail.