Well, in order to ask a question you need a question word (ĉu). The question here is actually two yes/no-questions connected.
The complete question in English would be "Is it blue or is it green?"
In Esperanto this would be: "Ĉu ĝi estas blua aŭ ĉu ĝi estas verda?"
Then you would drop the second "ĉu ĝi estas" because it's redundant. But you can't drop the first "ĉu" because that's the word, that makes the question a question.
It would be great if Esperanto could make a distinction between yes-no questions and multiple-choice questions, like Latin "-ne" versus "utrum... an" or Chinese 吗 versus 还是. Zamenhof slipped up there. Treating multiple-choice questions as a string of yes-no questions is clumsy.
As it says in the tips and notes section of Basics 2:
Ĉu introduces a (yes/no) question. The subject and verb are not inverted, unlike in English:
La kafo estas varma = The coffee is hot
Ĉu la kafo estas varma? = Is the coffee hot?
Oh, and in cases where no other question word like which/where/what etc. is appropriate, you will still need to use ĉu.
No. It's just a particle that turns things into a question. If you must think of it in English words, think of the phrase "Is it so (that)?"
La floro estas flava. = The flower is yellow.
Cxu la floro estas flava? = Is it so that the flower is yellow? = Is the flower yellow?
And because it's more like "Is it so (that)?", it can and does get used as a tag question:
La floro estas flava, cxu? = The flower is yellow, right?
And it can stand on its own:
La floro estas flava.
Cxu? = Really?
DuoLingo says both ways are valid. This is discussed in the the Adjectives lesson:
In Esperanto, adjectives are easily transformed into verbs, and are frequently used that way in conversations and in written texts . The most common form, however, is still estas + adjective :
Mi estas preta = Mi pretas = I am ready.
Mi estas malsata = Mi malsatas = I am hungry.
Do you mean ĝi and ĝis? There's all the difference. :D This words have nothing to do with each other.
The word ĝi is a third-person singular pronoun, used for an object or a being which doesn't have a gender or whose gender we don't want to specify.
The word ĝis is a preposition marking a point in space or moment in time, that is reached an action, but not passed over by it.
"Ĉu" is what's called an interrogative particle.
At the start of a sentence, it turns a statement into a question:
The flag is blue. = La flago estas blua.
Is the flag blue? = Ĉu la flago estas blua?
At the end of a sentence, it's a tag question:
The flag is blue, right? = La flago estas blua, ĉu?
The flag is blue, isn't it? = La flago estas blua, ĉu ne?
"Ĉu" is similar to the English auxiliary verb "do" or "did", or the "to be" verb at the beginning of a sentence.
"Li manĝas la kukon" means "He is eating the cake." but "Ĉu li manĝas la kukon?" means "Is he eating the cake?".
"Ŝi vidis sian patron" means "She saw her father" but "Ĉu ŝi vidis sian patron?" means "Did she see her father?"