Create a folder named optifine inside the .minecraft pack (should be like this: examplepackname.zip/assets/minecraft/optifine
after that create a .properties text file and name it color (Should be inside the optifine folder and look something similar to this: color.properties)
Inside the text document write "text.xpbar=ffffff", FFFFFF being the hex code, lastly save and close the document and you'll see that the exp bar number is colored!