When you select a data point such as a segment, line, or area, and apply a color from a think-cell menu, a think-cell message is displayed: "Your chart uses more colors than Graph can handle. One (or two, three, …) color(s) has (have) been replaced by Graph's generic color. Click here to learn more." Clicking the link in this message brings you to the current KB article.
Due to a limitation in Microsoft Graph it is only possible to reliably assign at most 24 different colors to the data points (segments, lines, areas, …) of a single chart. (If you assign the Office theme colors first, up to 28 colors may be used). If you exceed the number of available different colors, think-cell cannot assign the desired color to Microsoft Graph which uses some seemingly random automatic color instead.
Reduce the total number of colors used for data points in this chart. Your point may be communicated even better by emphasizing only some data points while the rest of the chart takes a back seat.
Please note, this only applies to charts that are based on Microsoft Graph. Other charts (e.g., Mekko Charts) do not show this problem, nor do think-cell features which are not part of the Microsoft Graph object, e.g., CAGR, difference arrows, or value lines.