The render seems a bit dark still. In the "indirect illumination" rollout; Increase the Final Gather "diffuse bounces" to 1.0.
Open the "environment and effects" dialog (8), choose the "physically based lighting, indoor daylight" preset, from the "mr photographic exposure control" list.
Enable the "photographic exposure" function, and set the "shutter speed" to about 90.0.
Increase the "shadows" function to about 1.0. This will invigorate the shadows.
In the "image control" group, increase the "whitepoint" value to about 9500.0. Note that this value worked best to capture the white points in the scene.
In the Physical scale group, enable the "unitless" function, and set it to about 120000.0.
Values between 90000.0 and 120000.0 work best.
While the daylight object is still selected, open the "modifier" command and increase the "mr sun basic parameter" "multiplier" to about 4.0.
The sun's color at sunset is yellowish; to emulate this, disable the "inherit from mr sky" function first.
In the "nonphysical tuning" group, increase the "red/blue tint" value to 0.4(equal to a yellowish/reddish colour). Also Increase its "saturation". To about 1.5 then test render it. (Fig.06)
The next step is to decrease the sky horizon height and change its color to a stronger blue (next to the windows).
Pan down to the "mr sky advanced parameters" rollout; and bring down the horizon line by decreasing its "height" to -1.5.
To change the sky's colour to a stronger blue, in the "non-physical tuning" group, decrease its "red/blue tint" value to -0.32. Then test render it again. (Fig.07)
The next stage is to add portal lights and a "fill" light in the foreground, to help improve further the overall illumination in the scene.
Select the top viewport. On the create command, open the "photometric light" set.
Click and drag the target light from the object type group to the front viewport, to create it. (Fig.08)
The principle is to create a nice diffused light to emulate light bounces around the designated area.
The first step is to change some of its default settings.
Now that the light direction is set, disable the "targeted" function, in the "general" parameters.
This will provide more flexibility when moving/ placing lights in the scene.
Change the "shadows" type to "ray traced shadows", in the "shadows" group dropdown list.
Ray traced shadows work best with mental ray.
In the "light distribution" type group, change it from "spotlight" to "uniform spherical"; to spread out the distribution of light.
In the "intensity" type, change it to dimming. This function will help control the light intensity.
Select and move up the light, near the camera's height (i.e. 1.186m).
Also, open the "mental ray message window", to monitor the rendering process.
Finally, change the "emit light from" function from "point" to "rectangle" type. Rectangle type is highly recommended to control the softness of the shadows (i.e. higher values of length/width= softer shadows). Test render. (Fig.09 and Fig.10)