riddle - I exist in the middle of the seven

I am a haven for those with thick walls

I challenge convention with my transparent skin

My effect is quite big, though I can be small

I keep the bad stuff out, and the good stuff in.

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What am I?

Answer

How about a

greenhouse

I am a haven for those with thick walls

like plants, whose thick walls refer to their stems Also, like BlueRaja-DannyPflughoeft suggested, plant cells normally have thick cell walls

I challenge convention with my transparent skin

greenhouses use transparent material (while plants are normally planted outside in the garden or in a house

My effect is quite big, though I can be small

greenhouses, even if they're small, can be effective at retaining heat

I keep the bad stuff out, and the good stuff in.

they can keep the bad stuff (chills, winds, bugs, etc) out, and the good stuff, like heat, in

EDIT: In regards to the title, with credit to user BlueRaja-DannyPflughoeft

the seven refers to the seven colours of the rainbow: ROYGBIV, where the middle is G -> green