They do get a lot of colouration but as they mature usually 6"+. But at that size should still be showing a little. May not be a redtail, maybe just a standard O. Goramy
the other one I have at the moment at a small size is about 4" and has much more red than the one above. Both showed obvious colouring at around 3" so were unmistakebly RTGG's at point of purchase. Fingers crossed yours comes through with some colour.
This is a later pic of the one pictured above at 13"-14" inches.
My males got colour at about 6" the females a lot later. I've got 4 females around 12" and still very little colour. You might catch glimpses of them in the 15k video. Their are 7 in the tank ranging from 12" to about 22/23".