I'll swap your beige walls for my green ones any day! Are landlords colorblind?
That said, I know how depressing it gets to have bland walls.
How long do you intend to stay there? That may affect the amount of $$$ you want to spend.
If you can, decide where your mirrors will go and get them up - not only are they a great way to reflect light but it will also feel more like "your space". (also mirrors leaning against a wall can easily break and picking shards of glass out of carpet is no fun, and takes ages
) Also re-arranging your furniture may make a big difference.
Adding color in the knicknacks or soft furnishings also works especially well against a boring wall color. Many agents advise people to paint their houses beige before selling as it is a softer colour than white and you can add almost any other colour in furnishings and still have it look OK.
You can also get cheap blank canvasses at many dollar store and it could be a cheap easy way to personalise things, especialy if you and a friend have a paint party