Brown is NOT a combination of all colors. It's a shade of orange. It's dark orange. That is an incorrect online mixer, it is not accurate. Use a CMYK color picker. CMYK stands for Cyan, Magenta, Yellow, and Key. Turn the last one, Key, to 0%. Key is the amount of shade/black in the color. Turn the first 3 sections, Cyan, Magenta, and Yellow, to 100%, that's the amount of each color. Cyan, magenta, and yellow create all colors. If you add 100 to each, it will turn black. I'll even supply you with a mixer that uses CMYK, RGB (Red, Green, Blue), HEX, Picker, and more.
https://image-color.com/color-picker
Using their Image Color Picker, I used the image you gave. The color code for that brown color you made is 0%, 26%, 43%, 39%. That color, brown, has no Cyan. It does have a lot of yellow and magenta, which make it orange, and it has some shade just because it's a darkish color. In other words, the color you made is literally dark orange, you can use ANY form of the widely used color pickers, RGB, CMYK, HEX, Picker, and such, and you can look up how the color codes work, it will prove to you that brown is dark orange, and that all colors combined together create black.