Find something to make using lots of colours I've embroidery floss. I have quite a few projects on the go, so I actually had to purchase 42 skeins the other day. That's ridiculous.
But if you take out the greys and browns, it's makes a pretty rainbow.
Purchase the cheap fabric bundles and make mismatched pillow shams.
This isn't colourful but it's weird. (It's not finished obviously) Strategically place weird things around the house to keep people guessing. Who doesn't need a little green man in a white sand terrarium so far only accessorized with rocks and a mini watering can? (I actually just can't decide what to plant in there....)
The fishbowl was $3.00. The sand was $2.00. The watering can and shovel were a few dollars.
Mirrors and plants and Momiji dolls and souvenirs and vintage cameras and colourful perfume bottles... although this photo is very devoid of colour. It's all there. So go with the suggestions and not the photograph. Also, the table cloth is covered with bright flowers and pineapples.
Make scrapbook paper pinwheels, glitter vases and tissue paper flowers. The one vase is filled with the letters L-O-V-E. I want to make a collage but I need a lot more letters.
Frame small pieces of Art or even Christmas cards. Make even more tissue paper flowers!
Use felt to add a bit of colour... flowers, garlands, plushies, whatever!
As an aside, colourful socks and tacky throw rugs can also brighten up a grey world.
Finally and most obviously.....
Open the blinds and curtains!