I have a bunch of webp images and need to convert them to png format. I know png supports different color modes like grayscale which I need.

Is there any Linux tool or online services to achieve this fast and easy?

I tried different online converters, but all of them convert webp to png with default RGB color mode.

This can be done on the command line using the imagemagick package. This has the advantage that it becomes easy to script for an entire directory of images (hint: either combine with the find -exec command or a bash for loop).

First install the package:

sudo apt update && sudo apt install imagemagick

Download an example webp image to test it out on:

wget https://www.gstatic.com/webp/gallery/1.webp

The imagemagick command:

convert 1.webp -colorspace Gray 1.png

You should now have a black and white png image in the same directory. -colorspace Gray converts the image to black and white.

For more info, try: man convert

Use GIMP 2.10.

click file>open and select open .webp file. Now go to image>mode>Grayscale Now go to File>export as and export as yourfilename.webp

and that is your image is converted to grayscale. While exporting it is upto you if want to select lossless or lossy.

Use Inkscape 1.1.1,

Open .webp file by click Open in File menu [File > Open]
Now click on Export PNG Image... in File menu. [File > Export PNG Image...]
On the side, in the Export PNG Image tab and Drawing sub-tab, click on the Advanced accordion.
Set Bit depth on Gray_1, Gray_2, Gray_4, Gray_8, Gray_16, GrayAlpha_8 or GrayAlpha_16 based on your needs.
Enter Filename with .png extension and click on Export button.

This will convert your webp file to grayscale png.

ffmpeg example

Using the format filter to set grayscale:

ffmpeg -i input.webp -vf format=gray output.png

See FFmpeg & Black and White Conversion for several other methods.

Convert an entire directory

Use a Bash for loop:

for f in *.webp; do ffmpeg -i "$f" -vf format=gray "${f%.*}.png"; done

Adapted from How do you convert an entire directory with ffmpeg?