onyenokwe,
First make sure the sun is actually shining on the side you are viewing via the 3d perspective settings (Ctrl+Shift+F3). Second the Sun's color can be controlled, this is like having a cool morning light or warm evening light from the Sun. Either way the Sun color will change all your colors.
The dull or flat look is just a general problem (ahem... "feature") with AC. You can improve it with reflectance and transparency. Try the AC training guides or buy one of Dwight Atkinson's books or my favorite go to
www.archicadtutorials.com.au and get the "Professional Rendering" DVD. it is for AC9/10, but the rendering engine in AC has not changed since then, so all is good.
All the best,
Snap
