This information applies to version(s): 2018, 2, 3, 4, 5, 6
Nik collection only supports images in the RGB color space, such as JPEG and TIFF.
Greyscale images can't be opened and will give an error message.
If you work with RAW files, you'll either need a compatible host software such as DxO PhotoLab, Adobe Photoshop, Adobe Lightroom Classic, or Affinity Photo in order to use Nik Collection as a plug-in or another RAW converter to convert your RAW files to JPEG or TIFF before processing.
DxO PureRAW can be used to convert RAW files from supported cameras to JPEG or TIFF for use with Nik standalone applications afterward.