To elaborate on the previous response, when a raster file is brought into ArcGIS for the first time it has no previously defined color maps/stretches, but ArcGIS tries to provide one by calculating statistics for each band and applying a stretch that will emphasize the important parts of each band. And, in ArcGIS Pro, it goes even further and uses “Dynamic Range Adjustment“ (DRA) so the stretch can actually vary as you move around and zoom in and out.
To “fix” this, select the layer and open the Symbology panel. Change the Primary Symbology to RGB, and set the Stretch Type to None. In addition, in the tab Statistics and menu Statistics, choose Dataset rather than DRA.
Or, use the Appearance ribbon, change the Symbology to RGB, and set the Stretch Type to None. Then if the DRA button is highlighted, click on it to turn it off.
See the attached images.
— Andy