It is always "allt" when it is used separately without a noun:
Allt är förgäves - everything is in vain
But with a noun, it is
all mat - all food
allt vin - all wine
alla människor - all people
"Alla" can also be used without a noun and then it means everybody:
alla var där - everybody was there
se rött is an idiom that means 'get angry', in that expression rött refers to the color red, and colors are called the neutral form when you speak about them like that – Jag gillar blått = 'I like blue'.
What you probably wanted to say is that you look red, not see red, and that would be Jag ser röd ut, and as you say, you are common gender so you're röd. Also if you just say Jag är röd 'I am red'.
That makes sense. Tack för förklaring! Getting the subtleties of the language is tough.
I was thinking about it and I realized that "I" am not the subject of the sentence, but the generic "It" is the subject. In that case if I said "It is blue" would I say "Det är blått" or "Det är blå?"
I think "att" is impossible here, because it is a conjunction, not a (relative) pronoun. It means "that" like in "I hope that the sun will shine tomorrow", not as in "the car that I bought" (only in this example, "that" is more or less synonymous to "which"). I've seen relative clauses introduced with "som" in Swedish. So, could one say "Det var allt som jag ville ha"? Edit: Or could one say "Det var allt vad jag ville ha"?
My understanding is that finns is the equivalent of exists and in English we can use is to mean exists (For example to say "There is a house in New Orleans" you are saying that a house exists in New Orleans so you would say "Det finns ett hus i New Orleans"). The Swedes are more strict in preferring to use finns. So with that to say "Det fanns allt att jag ville ha" you would be saying "It existed everything that I wanted" which doesn't make sense.
Also the past tense of finns is fanns. Am I correct native Swedes?
"det" = it (det regnar - it rains)
"det här" = this (det här är mitt hus - this is my house)
"det där" = that (det där är hans hus - that is his house)
Sometimes, if "det" is stressed, it is probably closer to "that" (jag skulle aldrig göra det - I would never do that).