Hi Kimmy - Welcome to PSG!
The hard part of your problem is selecting the pixels to be changed, not the process of changing pixels that have already been correctly selected. The latter is the easy part.
Can you post the image that needs to be changed, or at least a similar one? Without seeing it, we are working in the abstract with respect to selection techniques, and that leaves way too many bases to try to cover. For example, if your cool gray pixels occur in a relatively small number of fairly well defined areas (...like the blacks in a crossword puzzle. or cool gray lettering on a page) then selection tools based on geometry (eg, the pen, marquee, lasso and quick-selection tools) are probably the best choice.
In contrast, if your cool gray pixels are either in lots of small but well defined areas, or in areas with irregular, soft, noisy boundaries (... e.g., whispy cool gray hair against a blue sky background), then selections based on color would probably be the best way to go. Examples of selection techniques that are based on color include the color range tool, using channels to make selections, third party tools like Asiva Select, Vertus Fluid Mask, OnOne's Perfect Mask, Topaz ReMask, etc.
I look forward to seeing what sort of image we will be dealing with.
Cheers,
Tom M