Is this a vector image? Or did you import a raster image?
If it's a vector image, you should be able to select various objects and change their color and/or opacity (and/or just about anything you want to change). Object menu > Fill and Stroke, is the traditional way. But you could also use a palette or even filters.
If it's a raster image, you would probably be better off using a raster graphics editor, such as GIMP. However, Inkscape does have a large collection of extensions which could change colors (and do other things) for raster images. See Extensions menu > Raster. There are a lot of options there for changing colors. I don't know if there's one that will automatically make it b/w. I'm not very familiar with them. But you can experiment!
Beginners' Questions Pic to Blk & Wht
Inkscape.org
How to change a picture to black and white? Is ther a way to take different parts of a picture and make them lighter or darker?
Is this a vector image? Or did you import a raster image?
If it's a vector image, you should be able to select various objects and change their color and/or opacity (and/or just about anything you want to change). Object menu > Fill and Stroke, is the traditional way. But you could also use a palette or even filters.
If it's a raster image, you would probably be better off using a raster graphics editor, such as GIMP. However, Inkscape does have a large collection of extensions which could change colors (and do other things) for raster images. See Extensions menu > Raster. There are a lot of options there for changing colors. I don't know if there's one that will automatically make it b/w. I'm not very familiar with them. But you can experiment!