Hex #550000 or rgb(85,0,0) in RGB space is the tint of red. It has hue angle of 0.00 degrees, value = 33 and saturation = 100. #550000 can be obtained by mixing 2 colors: 1 drop of red, 2 drops of black.
Nearest safe hex - #660000. Below you can see the block with #550000 color and its structure: in procentage ratio and in drops of pigments. Click "TRY" button to move #550000 to the mixer and play with it.