delete a colour

bosshogg, is the logo an imported raster image (i.e. jpg, png, bmp,...)? Or does it contain vector shapes?

If it's a raster image, you'll be better off by either vectorizing it (learn about tracing bitmaps in Inkscape), or by using a raster graphics editor, like Gimp. Inkscape does allow for modifying a raster image's color by filtering (look into the Filters -> Color submenu), but you'll have more fine-grained control in an editor that is made to work with that type of data (pixel-based).

Else, if your logo is made up of vector data, there is the right-click menu in Inkscape 0.92 and higher which offers the option 'Select same -> Fill Color / Stroke Color / Fill and Stroke'. That way, you'd get all objects with the same color, and can change their colors all at once.

Or, the more sophisticated and professional way would be to use the named colors aka custom swatches. This would allow you to give each color you want to use in your logo a name, and assign the name to the objects. If you then, later, decide that you do not want e.g. 'Night Blue' to be as dark as you wanted in the beginning, you select Night Blue, and edit it to be brighter, and all objects that are in that color will change. See http://goinkscape.com/custom-color-palettes-in-inkscape/ for instructions.

Kind Regards,
 Maren