"I wear a yellow skirt in the spring."
It's because the iro on the end is unnecessary. No one says that. People have no doubt complained about the superfluous iro on the end of colours and they've corrected accordingly. If you tacked iro on the end of all colours it would be the equivalent of saying in English "yellow colour, pink colour, red colour" etc. You get the idea. In a way it's kind of condescending - like people won't know you're talking about colours unless you add colour to the end. It's superfluous and over stating. I'm glad they've listened to complaints and rectified them.
It is neither unnecessary and a lot of people say it when they want to put emphasis on the color. Furthermore the color is used when ascribing a color of a distinctive object, such as strawberry colored, 苺色. It is important to learn the iro for that particular reason, and also 色々 (いろいろ) is a relatively common phrase.
Apart from きいろい and ちゃいろ, いろ is not commonly tacked on the end of colours as Duo frequently does with eg. みどりいろ、むらさきいろ、ピンク色、オレンジ色. みどり、むらさき、ピンク、おれんじ already mean green, purple, pink and orange - aside from the arguable need for disambiguation between the colour orange and orange the fruit - what other meaning could these words have apart from colour? Also I never said that いろ or 色々 (which incidentally is an adjective which means various) are not commonly used words in Japanese. I have consistently maintained that Japanese words that depict colour (apart from the exceptions which I've pointed out) do not commonly have いろ tagged on the end.
This is genuinely wrong, and i asked a native japanese speaker. The correct way of stating in the springtime uses the particle "ni" when the sentence is constructed as such, not "ha".
[In the springtime][yellow-colored][skirt][i am/i will wear]
Using "ha" particle 春は黄色いスカートをはきます
[Specifically in spring][yellow-colored][skirt][wearing/will wear].
The significant difference here is that one states that in the spring they wear that type of colored clothing, while は particle means that they ONLY, EXCLUSIVELY wear that type of clothing in the spring.
There are two main ways of saying it in japanese, and there's plenty of connotations for which it's too early for most duolingo learners to get into. [colour] no [clothing] specifies explicitly, and does not use an adjective to describe the color of the clothing.
The best english "close enough" case would be the difference of "i want a yellow shirt" vs. "i want a shirt that is yellow", although in both sentences yellow is an adjective.
Because kiiroi is an i-adjective, while murasaki is not an i-adjective, even though it ends with an i, it's not an i-adjective. Only i-adjectives can modify nouns directly.
This is not english, the rule in your mind, that says, that any colour can modify any noun directly does not stand true here. That's why duo insist on different usage of murasaki when compared to kiiroi. Because murasaki cannot be used like kiiroi is used. It's a different type of word, they do not fit into the same category, therefore their usage is different.
I didn't say anything about adverbs so I'm not sure why you're bringing them up. I already answered your question. Sorry, I didn't notice that you were asking about two different words BUT my answer still stands - in your original question 黄色い is an adjective. It can modify nouns directly and doesn't need help. The 紫色however is a noun - the colour purple. It can be used to describe other nouns but it needs help - that is what the の is for. Nouns cannot modify/describe other nouns directly.
I meant noun adjective not adverb. Whiting on phone is tough. The question I am trying to get answered is why does Duo treat yellow and purple differently. This question (as well as I believe all other uses of 'kiiro' allows: "Haru wa kiiroi sukaato wo hakimasu" anywhere that purple shows up it insists on "murasaki iro no ..."