Anytime you think an image is too dark in AccuRender 3, try adjusting the exposure (Raytrace tab, click Adjust Exposure) to set the brightness and contrast and run the raytrace again. If you want an exterior to be lighter, set the Brightness to 0.25 and the Contrast to 0.5 as a starting point. This will lighten the whole image including the shadow areas and the areas in direct sunlight. Experiment with these settings.
If changing the exposure does not give you the image you are looking for, you can also change the sun settings. AccuRender 3 uses a very realistic sun calculation model. Sunlight comprises two elements: direct and indirect sunlight. Indirect sunlight is an estimate of the light reflected from the sky and ground.
By default, the sun has a cloudiness setting of zero, which can create dark shadows. The greater the cloudiness, the less direct sunlight the model will receive, causing lighter shadows. To experiment with increasing the cloudiness (click Lighting, then edit the Sun, Sun and Sky Settings dialog box, Settings tab), set Cloudiness to 0.3 to lighten the shadows.
Another method of reducing the shadow darkness is to change the sun intensity. To experiment with changing the sun intensity (click Lighting, then edit the Sun, Sun and Sky Settings dialog box, Settings tab), set Sun Intensity to 0.4. This will modify the ratio of direct to indirect sunlight. The smaller the sun intensity value, the lighter the shadows will look.
A traditional rendering method to lighten exterior shadows is to insert a second, opposing sun into the model (click Lighting, Add, Parallel). Locate this light source at an opposing angle to the sun's direction. To start with, set the Brightness of the parallel light source to 0.4 and the Shadow Darkness to 0.