What would cos my clutch fluid to turn grayish?

 

Air
Brake fluid absorbs moisture from air contact which causes it to darken in color

Clutch reservoir is really the only place fluid would be in contact with air, so there should be a black "cup" inside reservoir looks like this: https://i.stack.imgur.com/80sP9.jpg
It is removed and fluid is added then its put back in to lessen air contact, called an air dam, similar setup on the brake master but its attached to the cap

Clutch reservoir(and brake master) have Vented caps so fluid can flow out to replace lost fluid

You can leave the black cup out and fill reservoir, then go under the truck with a pan/container and open the slave's bleeder and let fluid gravity bleed from reservoir to slave, to get the old fluid out, or most of it
Shut bleeder and check reservoir you DO NOT want it to run dry, refill and open bleeder again until you feel all the old fluid is gone, by the color of the fluid thats coming out of bleeder