is it "grises" because the subject it's describing is plural and feminine?
what about "morne/mornes" ? doesn't that mean gray also?
Not exactly, its interpretation will depend on context, but it falls in the area of dull, glum, dreary, bleak, drab, etc.
So then why was "Les chats sont gris" correct? Or was that a mistake?
Why is "bottines" wrong?
So is "Les bottes est grises" also correct?
No, est is singular, sont is plural. So if you have des or les you need to use sont.
The boots are gay