Put an elastic band around your paint pot to wipe excess paint from your brush and it'll keep it from dripping down the outside of the tin.
Testing which paint to colour your walls? Paint on to wallpaper lining paper, instead of directly on the walls - this way you can move it around to different parts of the room without damaging your walls.
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Paint the edges of your doors a different colour to stand out. Don't forget to let the door dry before closing or it'll stick!
Quick update: Got a boxed frame or mirror? Paint the inner edges a different colour for a quirky look.
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Rental properties are often a generic shade of off magnolia, but that doesn't mean you can't add splashes of colour to your walls. Paint large sheets of MDF and mount them on the walls with removable hooks.