One of the internal color values, which is dependend from the choosen bar style and from the series index, where the first added series has the index 1, the second the index 2 and so on.

To set the default value for the color you can either pass the constant Default (or 0) or ignore this property. Instead of one of the over 180 predefined color constants you can pass any other valid RGB value.
NOTE: If the Type of your bars is RMC_BARSINGLE, you may have for each bar a different color. In this case you must set Transparent as Color property. Additionally you can define for each bar an individual color with ColorArray, ColorString or SetColorValue. If you don't define a own color table, the internal color table will be used: because these are only nine different colors, they will be circular repeated if your series has more the nine bars.