red
赭 centers on a specific earthy red color, like red soil or ochre. Its meaning is simply that shade of red.
赭 combines 赤 (red) with 者, which likely contributes the sound. The character is associated with red earth or ochre.
The left side 赤 means red, and the right side 者 looks like a person. Imagine a person covered in red earth, making the color 赭.
For シャ, picture a red shawl (shawl -> シャ) dyed with earthy red ochre. The shawl's color is 赭.