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Given your sample image, here two versions, depending of what you want exactly

Version 1:

Mat image = imread("woman.jpg", 0);     //read grayscale version of image (1-channel)
cvtColor(image, image, CV_GRAY2BGR);    //rgb version, r=g=b
Scalar color = Scalar(0, 0, 255);       //define desired color
Mat mask = Mat(image.rows, image.cols, CV_8UC3, color);     //create a mask of such color
Mat result;
addWeighted(image, 0.5, mask, 0.5, 0, result, CV_8UC3);     //weights can be adjusted to needs

Version 2:

Mat image = imread("woman.jpg", 1);     //read color version of image (3-channel)
Scalar color = Scalar(0, 0, 255);       //define desired color
Mat mask = Mat(image.rows, image.cols, CV_8UC3, color);     //create a mask of such color
Mat result;
addWeighted(image, 0.5, mask, 0.5, 0, result, CV_8UC3);     //weights can be adjusted to needs

Results (original/version1/version2):