Learn Croatian: Red je dug. - The queue is long.

No, this is a "false friend"! In Croatian, red is a noun that means a "queue", "line", "row", or "order". The Croatian word for the color red is actually crven (or crvena).

Why does the word for "long" (dug) have no ending like -a or -o?

Adjectives in Croatian must match the gender of the noun they describe. Because red is a masculine noun (you can tell because it ends in a consonant), the adjective dug also takes the masculine form, which is the base word with no extra ending. If it were describing a feminine noun, it would be duga, and for a neuter noun, dugo.

I looked up "long" and found dugi. Why is it dug here?

Croatian has two versions of many adjectives: a "short" indefinite form (dug) and a "long" definite form (dugi). When you use an adjective after the verb "to be" to state a fact (like "The queue is long"), you use the short form dug. You typically use the long form when it comes directly before the noun to specify it, like dugi red (the long queue).

Could I change the word order and say Dug je red?