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• First: 2.18*10^-38 is completely different from 2.18^-38
• Second: There is no single continuous rgb-value. Its a combination of 3 single indepedent values (red, green, blue) and has to be treated as such.

If you want a continuous indicator of "color", you should convert the rgb-values to something like HSV and use the hue component (together with appropriate saturation and value of course). Read this documentation of the pcl::PointXYZRGB Struct to understand, why they packed rgb into a float and how to access the single rgb-values of the point. (Either by recasting and bit shifting or directly by using the .r, .g, .b accessors)