What do the colors on the building pieces mean?

The color of a placed building piece refers to its stability, and similarly, its durability. House building does not ignore physics and gravity in Valheim, and buildings need to be properly reinforced so they don't fall apart. This includes using log poles from core wood, and roof planks from regular wood. Additionally, cross beams on ceilings and X beams on walls can greatly reinforce your structure. Do note that there appears to be a soft height limit on buildings as well; no matter how stable you make your structure, stability will decrease as your increase in height.

Some of the colors don't mean much, but a few are notable:

• Blue: A building piece is resting on the ground, and is fully stable.
• Green: A building piece is well reinforced, and is stable.
• Yellow to Orange: A building piece may be reinforced, but these colors don't seem to mean much at this time.
• Red to Dark Red: Very weak. Might be okay for roof pieces, but floors, walls, and beams may not be stable, and could break at any time.

Something interesting to note as well, you can sometimes watch the stability of a piece change simply by looking at it with the Repair option of the Hammer selected (this is to get rid of any "pre build placement" clutter). The piece will change colors until it's final "resting" stability, or may break if it gets to Dark Red. This is almost a live look at where your stability needs shoring up.