It is always best to choose a neutral colour to paint a rental property. While you might love strong colours, stripes or bold wallpaper, a strong interior will provoke a strong reaction in people who view the property to rent after you move out. Bold strong colours can also be very difficult to paint over, so neutral is generally the most rentable.
If the landlord grants you permission to paint, but only using neutral colours and you would still prefer something splashier, show them samples of your intended colour scheme and offer to repaint it when you move out (back to neutral, and at your own expense).
If the landlord does not give you permission then don't do it. If you go ahead and paint walls without permission you may face losing your deposit to pay for painting the property again.
There are other ways that you could brighten up the walls of your rental property.
You can create a feature wall using interesting photos or artwork. Use all different sizes, with mix and match colour/style frames. Take a look at Pinterest and search for ideas on how to arrange art for maximum impact.
If the landlord doesn't want the wall full of holes, try using "Command Strips" which are produced by 3M to hang things securely without causing damage to the wall.
Large canvases and wall art pieces can be pricey so if you want to keep costs down you could cover a large blank canvas (available from art supply shops) with bold fabric or even splash some paint around to make your own art. You don't even need to be an artist to create a great, bold look. Click this link to see a really simple tutorial from the American DIY emporium Lowe's that shows you how you can use a squeegee to create your very own wall art. Now that is anything but neutral!
Use a decorative room divider screen to create a feature wall and disguise an old or ugly wall.
With a little bit of creativity you can have a space that is unique to you without actually causing permanent damage or change to the rental property.