Brown and Brown RGB refer to the same CSS swatch here. Sienna is a separate named color, with a more orange hue.

Brown RGB Color Code and HEX
Tone scale
Tints
Color Conversation
Color harmony
Dark text
2.37:1
White text
7.08:1
Brown RGB gives the web’s reddish brown reference, closer to brick than coffee.
#A52A2A
#2AA5A5
#A52A68
#A52A2A
#A5682A
#A52A2A
#2AA52A
#2A2AA5
#681A1A
#7C2020
#912525
#A52A2A
#B92F2F
#CC3636
#D14A4A
Brown and Brown RGB refer to the same CSS swatch here. Sienna is a separate named color, with a more orange hue.
