If you do not want to add fertilizer (in case you already have some in there) you can try using just sulfur, it acidify the soil, just a little at a time (2Tablespoons in 1 gal -you can google the conversion) and see if color improves. You can also try Epsom salts, magnesium deficiency can cause yellowing, it won't hurt to try.
My orientals in the sun look like that, then get worse but as much I I try I just can't change the weather here - it is a VERY bad year.
