Like the color of some roofing

• Bluish gray
• Dark bluish-gray
• Gray, in a way
• Having a dull blue color
• Dull blue-gray
• Bluish-gray in color
• Like overcast skies

Wiktionary Word definitions in Wiktionary
a. Resembling the mineral slate.

The Collaborative International Dictionary Word definitions in The Collaborative International Dictionary
Slaty \Slat"y\, a. [From Slate .] Resembling slate; having the nature, appearance, or properties, of slate; composed of thin parallel plates, capable of being separated by splitting; as, a slaty color or texture. Slaty cleavage (Min.), cleavage, as of rocks, ...