Paint is by far the most cost effective and dramatic change you can make to a space. That’s not to say that having a room painted is inexpensive; but dollar for dollar, paint will make the biggest transformation for your buck! You can go from dark to light, from neutral to colorful, from cool to warm; what ever you want. Think about it; you can have a space painted, put back all of the original furnishings and you’ll end up with a completely new room! It’s amazing what a difference it can make.
Take this billiard room for instance: Before it was dark & uninviting and as a result; it was hardly used.
(It was even worse with the lights off!)
(My apologies for the poor quality photos but it was actually darker in person!)
After, it’s warm and bright and now actually draws you into the space. 
We did reposition some of the accessories but all of the original furniture was replaced and no changes were made to the lighting, even the old window blinds remained. (You may also notice that the lights above the pool table are not on when viewing the “after” pics).
Picture your room with a bold new color, a relaxing spa like shade or go soft and sexy! 
Try painting an accent or feature wall. 
Or opt for a fresh clean palette.
Many paint manufacturer websites allow you to upload a photo of your room and show you how their paint colors will look in your space.
Note: Always get a sample of the color and paint swatches in several areas of the room before painting! You need to see how the paint will look in the day time and in the night time; in natural light and man made light!
That said, having a room painted is not as inexpensive as you may think, (even if you do it yourself), so you’ll want to make sure you choose the right color and finish before you start. That’s where a Professional Painter, Decorator or Designer can help. 
Professional Painters may not want to get involved in helping you decide on color but they can tell you what colors are popular and more importantly they can help you choose the correct paint finish for the space. 
A Decorator and a Designer will help you in choosing not only the paint color & finish but can also help with a lot of other details such as; transitioning colors between spaces or helping you create a cohesive flow from room to room. 
So consider hiring a “pro” to help you before you start, you’ll thank yourself in the end (or kick yourself if you don’t).