Please do not get or do anything until you have a plan. That will give you a unified and cohesive look. Brown and blue are a popular combination, so you may think about a light brown or light tan for the walls and ceiling. One color posibility is SW7513 Sanderling. I highly recomend that you search the internet for different colors and also go to a paint store and see the brochures. They show you rooms with different colors. Also, SW (Sherwin Williams) and other paint companies have online visualizers where you can see a color in a room. You have rather cool colors in your room. Therefore, more of a cool brown (such as a taupe) may look better than a warm brown. If you get a warm brown, it may clash with the mostly cool colors in your room. Just keep that in mind as you go trhough this process.

You can pick up the same brown and blue for the wall art. You can go a lot bolder and larger with the wall art. (go big or go home!!!) This is your home. Express yourself to the fullest extent. You can go nuts! I have classic base pieces with my couch and entertanment center. Then I go crazy with all large colorful wall art. One thing you may want to do is to discover what your interior design style is. Write (one at a time) different styles in the top HOUZZ search box. Examples are coastal, casual, rustic, transitional. Note what you like and what you don't like. Here is one example of wall art. https://www.art.com/products/p55392111956-sa-i4244107/francis-campbell-boileau-cadell-low-tide.htm?upi=Q1MYYPV0&PODConfigID=8880735 If you get art online, get just the print and have it framed locally (it's cheaper that way). Other places to get art are local artists, art galleries, and some colleges have their own art galleries. Some libraries exhibit local art.

Wood floors with an area rug would be an improvement in this space. If you don't want to go there, add drapes and a larger piece of art over the sofa. What you have is nice, it's just too small. On the wall opposite the sofa try hanging two pieces of art vertically to fill in the space. A larger, more substantial coffee table would also help. While this room doesn't look exactly like yours I hope it will help to convey my ideas.

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Removing the pale carpet will give you the opportunity to anchor your space with an area rug with some color and movement. Curtain panals will also be helpful for this and the acoustic challenges all that hardwood will bring to the forefront. The spindals do date the space, maybe something sleeker in design. Not sure if the railing is necessary or not.

Ways to make room cozier.

Grounding/balancing room:

Suggest a larger/more substantial coffee table and pulling the chair in will help create a cozier, more inviting feel. Try moving the side table on left of love seat to other side and add a large lamp for more balance (see last picture). Move floor lamp to other end of couch and the plant where the small side table was. A cozier room example (ignore furniture choice).

Accessorizing:

Look at inspiration rooms and save the ones you like that contain elements you are drawn to. If you start with the rug, other choices will be easier. Blinds would warm up the space, suggest larger art over couch, and large vertical art on the opposite wall.

It appears the railing is due to the living room being step down/ step up so rail is needed but if you can replace the spindles do so. the room is nice & tweeks such as a lamp (tall table lamp substantial enough for room) on corner end table & rethinking the art work.

I think the room is lovely and just needs some tweaks! I would get rid of the small end table by the half wall and instead put a lovely console table there and create a vignette with a pretty picture above it. I think a few touches of pale pink/blush would brighten the room - a big vase of flowers, throw blanket, books bound in blush linen, marble objets, etc. And add another small green plant or two.
I agree with others about needing a larger coffee table to ground the space. Maybe a warm wood one and not glass as the room seems to want something a bit heavier in the middle!

Here are some ideas for working with your color palette!

I would just start with a vibrant color on some of the pillows and the throw and see if that satisfies your desire for more punch.. I think it is a beautiful room.

Easiest fixes first:

Much larger artwork above the sofa.

Pillows that incorporate light blues and navy, some with texture.

Floor plant with broad leaves - trust me it makes a difference in the space!

Beefier coffee table.

Next level fixes:

Window treatments, Yes, they make them for moon shaped windows too.

Hardwood floors and an area rug.

How is this room lit at night? Is there overhead lighting we are missing?

I would get a bigger picture for over the sofa and even hang some other framed pictures behind the sofa. Or maybe even a beautiful framed round mirror. I think the room needs some pop===some nice accent color, perhaps even an area rug in front of the sofa. I suggest you consider any one of the following for an accent color in pillows, or a throw: dusty rose, mint green, turquoise, lavender, plum, navy blue. I would also get one or two more large exotic plants for the room. I would also get a bigger coffee table than the one you have.

I think you just need more color. I'd go with a darker and/or brighter shade of the sofa color. and get a small area rug to put under the coffee table.

this is a pretty room! Such good suggestions above -

(FWIW my sister has pale blue sofas and has this hung above one on a large wall and it looks terrific -it's huge and has great texture, which I think would benefit your room - you may not like this but just wanted to give an idea)

If removing the railing, then you'll be able to place the chair closer to the sofa. It looks like you've done a good job, maybe look at adding some textured white pillows, along with larger artwork over the sofa. Coral can go well with a light blue and teal colors, along with white and cream. Here are some visuals to aid you more:

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The reason you feel your room is "dull" is because of several issues. There is nothing anchoring the space, like a colorful rug, paint, proper size tables, art. I agree with you to eliminate the railing as it's not the style of the rest of the space and doesn't seem to serve a purpose.

Consider tightening up the space creating a cosy and inviting plan.

Play up the window wall by concentrating on that elevation. First by adding a window treatment outlining and combining the windows. (see sketch below) Have a custom upholstered valance from wall to wall, edging into the top of the window frames and add curtain panels.

This floor plan rearranges the furniture by centering the large sofa in front of the arched window, add a long sofa table behind the sofa with two large lamps. Don't worry about the lamps being framed by the windows or the sofa seeming too large. You will see that adding other pieces will create interest so your eye will travel around the room.

The large wall where you now have the large sofa should be treated abstractly and not your typical layout. This may seem counter intuitive, but a well designed room ignores the walls. When you look into the room, think "tight vignettes". On the left of the lounge chair add a console or cabinet with a large colorful piece of art above and an arrangement of accessories. For the art, I strongly recommend supporting. your local artist community.

Behind the loveseat add a low long ottoman, maybe pile books.

The tables should be larger especially the coffee table and introduce wood.

Here's some good shots of ottomans behind sofas.