Translation:I do not want a purple skirt.
I think in this case it's different. Murasaki iro is not a "na" adjective, but the concept of "purple color", since "iro" means "color".
So you can think of "murasaki iro no sukaato" as "a skirt of purple color", which makes sense, even if it's not the conventional way to say it in English
Others have said similar but here's my two cents:
"むらさきいろ" (紫色) isn't really an adjective like "purple" in English, it's more a noun like "the colour purple".
It might be stretching it a bit, considering the differences between the languages, but the way I see it you can't have "a the colour purple shirt" (むらさきいろシャツ), but you can have "a shirt of the colour purple" (むらさきいろのシャツ).
きいろい is a much older color word that purple. Modern Japanese doesn't really let you create new -い adjectives, especially not by just adding い (there are a few specific suffixes like -らしい that can be added to nouns to make new adjectives), but when the word きいろい was formed, you could make new い adjectives.
Excuse me, I'm confused... Is there any difference between "skirt" and "skirts" translation for the word "sukaato"?
At one translation I was corrected just writing "skirt". But in another practice it's considered as incorrect, I had to use "skirts" as the correct translation.
Is there any explaination why is that?