Which of the following is NOT a factor influencing the choice of scale for a map?

• The size of the area to be mapped directly determines whether a large or small scale is needed. A larger area requires a smaller scale (e.g., 1:1,000,000), while a smaller area allows for a larger scale (e.g., 1:10,000).
• The purpose of the map influences the scale. A map for general navigation will have a different scale than a detailed city plan or a geological survey map.
• The level of detail required is directly linked to the scale. A larger scale map can show more detail than a smaller scale map.

Options A, B, and C are all valid factors that influence the choice of map scale.
Option D, the availability of colors, is a factor related to map design and aesthetics, but it does not determine the mathematical ratio of the map to the ground (the scale). The scale is chosen based on the geographical and practical requirements, not on the color palette.

Based on the analysis, the availability of colors is not a factor influencing the choice of scale for a map.

The correct answer is $\boxed{\text{D. The availability of colors}}$.