Translation:Una cartera marrón.
I have never heard anyone say "marrón" for brown. I've heard "color café" and "moreno/a" all my life.
why can't it be "una cartera de marrón"? when should i only put "de" for adjectives?
Generally you don't, and for sure you don't need "de" with colors. I'd say "de" is more like "of" in English and goes rather with nouns than with adjectives: "la plata de madera" is literally "the plate [made] of wood", "la falda de mi madre" is "the skirt of my mum"
You use un when it is a masculine word and una when it's feminine. In this sentence, the purse is feminine so you'd use una. (Ex: A brother = Un hermano while A sister = Una hermana; notice the "o" at the end of hermano. This means it's masculine. The "a" at the end of hermana means it is feminine. Same with a purse. Bolsa/cartera is feminine so you'd use "una"). I hope this helped!
I'd be cautious with focusing on the endings to determine the word gender. Sure, they often work that way and it's helpful to notice such patterns, but you have to remember it's not the rule - there are examples that don't match, like el problema, el mapa, la foto, la moto