Common alternatives for Congo red include: 

• Sirius red F3B: The most common alternative to Congo red, Sirius red F3B is darker red in color and is preferred in some applications because its darker coloration is easier to see. 
• Benzo Scarlet 4BNS: The color of Benzo Scarlet 4BNS is similar to that of Sirius red F3B. 
• Sirius Scarlet GG: This dye has a slight yellow tone.