Sapstain fungi attack living trees but are typically only successful in establishing colonies in weakened or dying trees.

Sapstain spores are often introduced to live trees by beetle activity. Spores which have attached themselves to the beetle's bodies come loose as the beetle move s through bark and wood. After germination, the growing fungi penetrate deep into the sapwood. They typically cause no structural damage like reduction of bending or compressive strength. 

Beetle trails in Lodgepole Pine.

Both sapstain and bluestain fungi feed on sugars and carbohydrates (tree food) contained in the cavities of wood cells instead of on structural materials in the cell walls. Because much of this food is stored in ray cells, which radiate out from the center of a log, stains caused by sapstain fungi appear to radiate in toward the center of the log as seen in the photo below.

Source

A log end affected by sapstain fungi.

By emptying wood cells of their contents, sapstain fungi make wood more porous, which increases its capacity to absorb water. Increased water absorption can raise wood moisture content (MC) to levels at which decay fungi become active, so although they don't actually decay wood, sapstain fungi create conditions which encourage its decay.

Staining fungi actually may appear as blue, purple, grey, brown, yellow, orange and sometimes red. 

MOLD FUNGI

Mold fungi are typically found growing on the surface of wood or wood finishes and are often easily cleaned off. If they’re allowed to grow long enough, they may create stains which require brushing or planing to remove.

Mold growth caused by snowdrift which accumulates on the downwind side of homes.

Courtesy of Permachink

Surface mold colonies growing on bare wood.

Mold fungi can also grow on inert materials such as plastic and drywall where they consume dust and dirt. Under the right conditions these colonies can grow very quickly and the high concentrations of spores they can produce in indoor air have been known to cause health problems, sometimes very serious problems, in those with asthma, allergies, lung disease or weakened immune systems. 

All photos not cited are by Kenton Shepard