Learn French: Ces algues sont toutes vertes. - These seaweeds are completely green.

In this sentence, tout is acting as an adverb meaning "completely". Normally adverbs never change form to match a noun, but there is a special rule for tout: when it comes before a feminine adjective that starts with a consonant (like vertes), it must agree in gender and number. Since algues is feminine plural, the adverb becomes toutes.

What would happen to toutes if the adjective started with a vowel?

If the feminine adjective started with a vowel, the adverb would ignore the gender and remain the default tout. For example, if we used oranges instead, the sentence would be Ces algues sont tout oranges. The special agreement rule only triggers before feminine adjectives starting with a consonant or an aspirated "h".

Does toutes sound different from tout, and how do I know it means "completely" instead of "all"?

Yes, you pronounce the final "t" in toutes (it sounds like "toot"), whereas the final "t" in tout is silent. As for the meaning, while toutes les algues at the start of a sentence would mean "all the seaweeds", placing it directly before an adjective like vertes after the verb usually makes it an adverb meaning "completely" or "very".

If I didn't know the word algues, how could I figure out its gender from this sentence?