Learn Italian: Questi fiori sono viola. - These flowers are purple.

Viola is an invariable adjective, meaning its ending never changes to match gender or number. This rule applies to several colors named after nouns in nature, like viola (a violet) or rosa (a rose). Whether you have one purple flower or ten, the word stays viola.

Why do we use questi instead of queste here?

We use questi because fiori is a masculine plural noun. In Italian, singular nouns ending in -e, like fiore, change to -i in the plural regardless of their gender. Since fiore is masculine, its plural requires the masculine plural word for "these", which is questi.

Wait, doesn't sono mean "I am"?

Yes, but it also means "they are"! The verb essere (to be) uses the exact same form, sono, for both the first-person singular (io sono = I am) and the third-person plural (loro sono = they are). Context always tells you which one it is, and here it matches the plural subject questi fiori.