Which factor is least important when evaluating the suitability of fill material?

When evaluating the suitability of fill material, the least important factor is the color of the material. While color can sometimes offer insights into the type of material and its properties, it does not significantly affect the performance of the fill in terms of structural integrity, drainage, or compaction.

In contrast, other factors such as compaction capability, moisture content, and compatibility with existing soil are crucial for ensuring that the fill material serves its purpose effectively. Compaction capability relates directly to how well the material can be compacted to achieve the necessary density, affecting stability and load-bearing characteristics. Moisture content is vital since it influences the material's workability and compaction. Compatibility with existing soil ensures that the fill will interact positively with the surrounding soil, preventing issues like settlement or erosion.

Therefore, while color may provide some information, it does not play a significant role in the engineering aspects of fill material selection.