You can't give a Radar image the same properties as an optical image. The wavelength used for producing the different images doesn't give you the same information at all.
You "see" a similar image because the electrical signal received by the captor is converted to a digital numerical value and then to a grey value in a n bit depth so it can be displayed on a computer screen. But you actually see the same scene (compared to an optical image) with a completely different perspective. Those two electromagnetic waves operates in a range of wavelength that are physically very different and thus have a completely different behavior with it's environment.
But if you don't care about the information in the image and you just want to "colorize" the image only for a visual effect, maybe you could pansharpen it with an optical image. But then you could simply use the optical image directly if the spatial resolution is adequate for your needs.