When designing in xib files in Xcode you might want to use RGB colors, which can be hard to find. It is possible you just have to know where to look.
Select the item you want to set the color code. Then in the toolbar select the color (here it’s the color of a toolbar and called tint). In the new windows “Colors” select the tab looking like a color slider. In the dropdown select “RGB Sliders”.
Here you can set the Red, Green and Blue color.