Lumber is a porous material.  Wood swells when it absorbs moisture from the rain or snow and shrinks when the sun's heat releases moisture.  The drying process creates stresses in the wood which are often relieved by warping, splitting and cracking.

Wood takes years to dry but wood is installed before it is dried out.  Depending on the installed climate, wood cells lose their natural fiber content to the air which can cause shrinkage and movement in the wood.

Many species turn gray and some turn silver or almost black.  This is a natural reaction to the aging process.  

Every piece of wood is constantly taking on or giving off moisture in an attempt to read the same humidity level as the surrounding atmosphere.  During long, dry periods, boards will shrink leaving gaps.  Additionally, during periods of heavy rain, the wood expands and gaps will close.  

All treated lumber, including water-repellent lumber, will internally dry during the first 60 to 90 days after installation.  This drying process will lead to shrinkage and some checking on the boards.

When nails are driven into a piece of wood, usually the hammer knocks off the galvanized coating on the head of the nail.  Eventually, the streaking caused by the rusting of the nail head will blend with the weathered gray of the wood.

Your wood fence needs about 2-3 weeks to dry-out in optimum conditions before it can be painted.  Painting can increase the longevity of your fence.  Your paint will need to be touched-up annually to remain pristine otherwise biennially will be sufficient.

Since pressure-treated wood is treated with chemicals it usually takes about 2-3 weeks to dry out depending on the climate.  Staining does not provide the same amount of protection as painting does.  Stain will need to be touched up annually for your fence to remain pristine.