What's a sweater? Do you mean jumper?
No, they mean sweater. Jumper is джемпер.
"Jumper" is what they call a sweater in the U.K. In American English, a jumper is women's garment that is a combination top and shorts in one garment.
Why is a jumper male with ий?
singular masculine adjectives end in ий. светр is masculine, so it takes the masculine adjective червоний. машина is feminine, so "red car" would be червона машина. яблуко is neuter, so "red apple" would be червоне яблуко