Oh snap! Good point.
Another thing to consider: if someone had silver colored blood, their color would be different. Especially lighter-skinned people, but anywhere that's normally red would look different no matter the ethnic skin color.
For instance, many people have pinkish lips. The lips would cease to be pink. Which, of course, could be covered with lipstick, but if it's a guy, that's definitely a questionable sudden change when one knows Shapeshifters are in the picture. Women could also use make-up for other areas of exposed skin which would normally be red.
As for me, my hands are really red pretty often. I don't know if anyone would notice that not happening anymore, but they'd certainly notice my hands being gray every time I wash my hands with hot water.