I understand and agree with the concept of a Muslimah's dress in public being simple, but is it really required to wear plain clothing with no designs and so on it? I am obviously not talking of big, bold, colourful designs, but is there any evidence that requires plain garments that do not have any flowery designs or dots/stripes to be worn by women?

All perfect praise be to Allaah, The Lord of the Worlds. I testify that there is none worthy of worship except Allaah and that Muhammad is His slave and Messenger.

It is known that among the objectives of the sharee'ah in obligating women to wear the hijab is to prevent men from being seduced (tempted) by women because of the inclination by instinct that Allaah created in men towards women. It is for this reason that the scholars conditioned that the hijab should not be an adornment in itself.

Shaykh Al-Albaani said in his book Hijab Al-Mar’ah Al-Muslimah fi Al-Kitaab was-Sunnah (the Hijab of the Muslim Woman in the Quran and the Sunnah):

"The second condition is that 'it should not be an adornment in itself', as Allaah says (in the verse which we have already mentioned in Chapter 24 - Al-Noor) (what means): {…and not expose their adornment.}; in general, it includes the apparent clothes if they are adorned in a way to attract men. The evidence about this is that Allaah says (what means): {And abide in your houses and do not display yourselves as [was] the display of the former times of ignorance.} [Quran 33: 33]

Moreover, the Prophet said, 'There are three kinds of people: do not ask about them: a man who abandoned the jama'ah (group) and disobeyed his Imaam (leader) and died as such (i.e. disobedient), a slave woman or a slave man who fled from his master and died as such, and a woman whose husband had fulfilled all of her worldly needs is absent from her and who does not cover herself properly [in his absence]; do not ask about them.”

Displaying or not covering properly or not wearing the hijab is for a woman to show her beauty and adornment and what she is obliged to cover of herself, which incites the desire of the man.

What is meant from the order to wear the jilbaab is to cover the beauty of the woman; so obviously the jilbaab should not be beautiful in itself.”

Finally, we are not aware of any evidence that differentiates between what you have mentioned as big bold designs or small designs as long as it is an adornment that men can see.

Allaah knows best.