By default, the S9 comes in "Adapt Display" mode, which attempts to adjust saturation and other display parameters depending on conditions. However, by default, this generally means it uses a color gamut that's much wider than sRGB, which produces oversaturation on content meant for sRGB and since Samsung doesn't appear to be using Oreo's built-in color management API, it doesn't switch on the fly.
By jumping into display settings and changing the Screen Mode to Basic, this forces the phone to display colors within the sRGB gamut. This reduces overall saturation but should increase accuracy, especially when viewing content made in sRGB.