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copyTo is the correct approach, however be mindful that the function will fail without warning in many cases. A common pitfall is if you try to copy an RGBa image into GBR or vice versa

var selectedArea = RGBaImgToDisplay.submat(resultRect)
sampleImageColor.copyTo(selectedArea)

will not work, unless you first make sure that sampleImageColor is also an RGBa like so:

cvtColor(bgrImage, sampleImageColor, Imgproc.COLOR_BGR2RGBA);

Then it works as expected