This tutorial will show you an easy way of painting a room, in photoshop!
Ok, so if you got a picture of a room and you think the colour perhaps sucks quite a bit/you want to change it, this tutorial will show you how to give it a quick makeover, alot easier than it would be with real paint.

Open your image, i'm using this one below that I simply found on Google, if you'd like to use it as an example, simply left click, and then right click to copy from the new window.

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Using the Pen Tool (), i'm going to go over all the walls of which i'm choosing to 'paint'.
Use the following images as an example of I'm doing this.

Using the corner of the walls, you can begin to see how I am covering certain areas:

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You should now be able to see a good idea of whats happening here now:

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Things may start to get confusing, but don't worry it will take shape.

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Having now covered the whole wall area we should with left with something like so:

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Now, to the colouring, we can either adjust colour, and lower the opacity of our new shape (made by the pen tool).
E.g. I've set the shape opacity to 31%, and put a colour overlay of #014189.
This left me with the following:

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However (But not as good) we could also hold control (Ctrl) and click our layer to get a selection, like below:

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We now want to copy the selection (Ctrl + C), then paste (Ctrl + V), this is so we can adjust the Opacity later.
Then with with our copy selected (hold Ctrl and click the layer), we want to hold Ctrl + U to to get the Hue/Saturation up.
Select Colorize and play with the options.
With the following settings:
Hue: 207
Saturatuion: 49
Lightness: +2

Layer Opacity: 56%
I created the following:

(Click To See Fullsize)

Compared to the other method, the first looks better, has less pixelation, and is much easier.

Choice is yours!
WE'RE FINISHED!