Take a hacksaw to "empty" ink cartriges. Use plastic gloves . Pour remaining inks into little bottles.I use empty saffron bottles or ones previosly home to food dyes. Mix in a saucer pallette. You can work out CMYK ratio from colour info readouts. Spot with a top quality 000 or 0000 brush.Test colour and depth on a scrap of unprinted paper the same as the one to be spotted. Dilute with water containing a drop of detergent. Spit works too, as old time retouchers know.