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As StevenPuttemans suggest you can use applyColorMap,

Or in another way use HSV as color space , and use hue value for color range,

See an example,

 cv::Mat hsv(200,180, CV_8UC3, cv::Scalar(0,0,0));
 for(int x=0;x<hsv.cols;x++)
 {
     int h=x;
     int s=255;
     int v=255;

     Scalar color;
     color.val[0] = uchar(h);
     color.val[1] = uchar(s);
     color.val[2] = uchar(v);

     hsv.col(x) = color;

}

 Mat bgr;
 cvtColor(hsv,bgr,CV_HSV2BGR);
 imwrite("hsv.png",bgr);

As StevenPuttemans suggest you can use applyColorMap,

Or in another way use HSV as color space , and use hue value for color range,

See an example,

 cv::Mat hsv(200,180, hsv(100,180, CV_8UC3, cv::Scalar(0,0,0));
 for(int x=0;x<hsv.cols;x++)
 {
     int h=x;
     int s=255;
     int v=255;

     Scalar color;
     color.val[0] = uchar(h);
     color.val[1] = uchar(s);
     color.val[2] = uchar(v);
      hsv.col(x) = color;

}

 Mat bgr;
 cvtColor(hsv,bgr,CV_HSV2BGR);
 imwrite("hsv.png",bgr);