When I want a different look for the room, I move the pillows around to another seat or I purchase new pillows. I recently pulled out my Mama's Singer sewing machine (older than me and I'm 50) and sewed pillow covers.
Most sofas will come with two matching accessory pillows or two matching pillows and two contrasting pillows. This look is fine, but you are limited to the design or look of the furniture manufacturer. There's no rule to say that you have to use the pillows that came with the sofa.
When I purchased my current sofa in 2006, I ordered four accessory pillows--two of the same fabric as the sofa and two made from the fabric of a contrasting chair. This is the way the sofa look when I first got it.
I initially wanted a monochomatic look so I wanted the colors to be very neutral and muted. I lived with this look for a few years, and then I decided to spice things up a bit. I still have the two pillows that match the sofa fabric, but I added two pillows that I purchased from Dreamweavers and I made the giraffe print bolster pillow. See how the sofa has been transformed below:
By adding a pop of color and bold pattern, the sofa has been perked up.
Chairs don't normally come with pillows. If you have your furniture custom made, you can have pillows made for the chairs. But again, you are usually limited to the fabric choices the furniture manufacturer has in stock.
Notice how plain this chair looks without a pillow:
Now see how much better the chair looks with a bolster pillow:
For a brighter look in the summer months, I found this pillow at Target:
See how changing out pillows in this chair gives the chair a totally different look.
The pillows you select do not have to match the chair fabric. I try to find pillows with some of the same colors as the chair or sofa fabric but with a totally different fabric. This adds a layer of texture to the room.
If you just can't deal with different patterns, you can still add some interest to your furniture. I helped a friend to select the fabric for her custom made sofa and chairs. She had two accessory pillows made for the sofa out of the chair fabric. We then went to Bed, Bath, & Beyond and purchased pillow covers to match the red in the sofa. A pop of red is just what this room needed.
If you want to change the look or mood of your room, try changing out the accessory pillows. Don't be afraid to mix patterns. Just make sure the pattern in the pillow picks up the color(s) in your sofa or accessories in your room. Go for it!!