I am having some trouble making an image greyscale. Its an OS map and i dont want the colours to show since i will be highlighting certain areas with transparent colours. I tried Extensions>coulor>greyscale and it said "greyscale working" then the image dissapeared. I have tried to find something under the filter menu but the best i could find is "blueprint" but i dont really want to use this since some of the transparent colours i will be using are blue.

Am i missing something obvious?

Color extensions are supposed to work on vector objects. To handle raster images you need to use the Raster extensions

Could you be a bit more specific on which extension to use? I tried a few but none did anything like greyscaling. I want the colours of the image to be in greyscale. thanks

Ah sorry it's true raster extensions can/should be used on raster images but there doesn't seem to be any desaturation extensions.

You can do this effect though by using filters, select your object and go to Filters > Color > Desaturate.

Don't forget to use the PixelSnap extension or manually snap your image to pixel grid so it's not blurry when exported.

There are probably a few different ways to do it, but here are the first couple I stumbled across:

HSB Adjust - set the Saturation to -100
Channel - Depending on the source image and the channel you pick, this may or may not be suitable

You could also use the Desaturate filter (from the Filters menu), or alternatively use an external tool, such as The Gimp, to desaturate the image before you import it into Inkscape.

Edit: Beaten to it on the Desaturate filter

Thanks guys i went down the desaturate filter, cant believe i missed that. Just a question about the pixel snapping, when i apply it it says "pixel snapping working" but afterwards the menu for pixel snap is still shown. Just checking it has indeed pixel snapped since there's no visual sign of the pixel snapping.

For irregularly shaped objects and objects without a lot of vertical and horizontal lines, pixelsnap will not show much effect, and for raster images the effect will be mostly on the edge. Anyway the effect really is for export purposes, so export a butmap to see the effect before and after.
If you really want to see it on screen, you can make a pixel unit grid and see the object move just a little to line up with the grid.

Cheers, was just wondering cause in the end i'll be saving as a png file for printing and was wondering if the image would be blurred as prkos said. will pixelsnap anyway to be on the safe side.

I had the impression that pixelsnap was for images used on webpages. Does it really help at all for printed images?