Roblox round is part of the default styles that are provided (which are ugly), so I do not think you can edit the color simply by changing the RGB property (which is dumb). I suggest using 9-slice + a rounded square image to reproduce a cleaner version of this where you can just change ImageColor3.
There is actually a way to change the colour of the RobloxRoundButton with the UIGradient instance. I sometimes change the colour of the RobloxRoundButton (I use RobloxRoundDropdownButton, since it is white.) and then add a gradient to it.
I know there is another way, but I figured I’d mentioned this so everyone knows they can change the colour of the default Roblox buttons if they really wanted to.