Firstly, if the house you live in is rented or leased, you will need the permission of the owner to do anything and it’s probably better to get that in writing, even if you just want to paint the front door.
If you own the house, but it’s Listed, or situated within a Conservation Area, the Broads or a National Park, you may well need permission from the local council to paint your front door or erect a flagpole. Also, if you are intending to fly a flag near a pavement or road, there are restrictions imposed for driver safety, so that the hedge, flag, gate doesn’t impede vision or cause a distraction.
But if you don’t have those restrictions, then you can fly a flag on a flagpole or from a pole attached to your house or on the roof
The European Flag gives the history
EU flag technical specifications and colours – Pantone Reflex Blue and Pantone Yellow
If you fancy an EU flag feature interior wall, it looks like you will have to get the Pantone Yellow mixed, but there does appear to be a Pantone Reflex Blue eggshell.
Crown Trade Acrylic Eggshell Pantone Reflex Blue.
Doing a bit of a Banksy on an exterior wall, will need planning permission for anything bigger than 0.6m according to the Planning Portal. But repainting the shed is fine, so too is festooning the garden in EU flag bunting.
In Southern England, it’s possible to make dreams of a Mediterranean-style garden a reality, complete with grapes, figs, olives, oleanders, bougainvillea etc. A good starting point is The Mediterranean Garden Society