Grey colors can come in a lot of variations. People who have renovated a house know that grey can be a bit green, red, blue, brown. If you are using black & white images in your presentation and you see one that is off-grey, it is better to force its color to be pure grey so it is consistent with the rest of the presentation.
Select the image, click format, select re-color and pick a black and white color scheme for the already almost grey image.
Photo credit: Matt Clark