Difference Between Sapwood and Heartwood
Sapwood
You can usually spot the difference between the two distinct parts of the tree’s xylem if you were to view a crosscut of a mature hardwood tree’s trunk. Once you remove the comparatively thin outer layers, the tree is essentially made up of the sapwood, which is lighter colored wood, and heartwood, which is darker in color.
Sapwood is new wood and is like a pipeline which moves water through the tree up to the leaves. Essentially the working component of a tree, sapwood transports water and sap similar to the way blood flows through our veins, capillaries, and arteries.
Lumber cut from sapwood shrinks considerably as it goes through the drying process. This part of a tree is also far more susceptible to fungus than the center of the trunk.
Heartwood
As new rings of sapwood grow, the inner cells are put out of commission and become heartwood. The central, strong pillar of a tree is heartwood. Although this portion of the tree is dead or retired sapwood, it doesn’t decay. As long as the tree’s outer layers are intact, heartwood remains strong.
What occurs as moisture is no longer transported through the straw-like cells, the pores in Heartwood are filled with organic material. The presence of chemicals called extractives causes the cell walls in heartwood to change color. Extractives are what create the rich color and unique character in the heartwood.
The functional beauty of heartwood for a woodworker is threefold. First, it’s significantly less susceptible to fungus. Secondly, heartwood contains far less moisture than sapwood and will have far less shrinkage when it’s dried. Thirdly, heartwood becomes in some ways as strong as steel as the fibers are bound together.
Source: https://www.wagnermeters.com/sapwood-and-heartwood/
Conclusion
Hence, the darker variations on solid wood simply means they are the heartwood of the timber. Do not demean their quality based on their color, but instead appreciate the fact that they are comparatively stronger and more resistant!