So basically the only colours allowed are orange, brown and purple

Here are some candidates:
Bhutan: Is mainly two shades of orange, the dragon can be (with some imagination) described as greyish.
Qatar: Assuming that the white doesn't count, this flag is just brownish.
Vatican City: Looks like Yellow and White, but officially (long heraldic history) it is Gold and Silver.
I will think some more on this.