Get a deposit-only hair dye
You need to buy a deposit-only hair dye which matches your base hair color. Deposit-only hair dyes are widely available and considerably less harmful to your hair compared to regular hair dyes.
It is recommended that you go for the darkest color that matches the color you dyed your hair. Check the color of your roots and make sure that you match it with shade indicated on the box of hair dye.
Mix the dye in a bowl
Deposit-only hair dyes also need to be mixed before application. Just take a bowl and follow the instructions on the color box regarding mixing. For safety, wear gloves and use a small brush to mix the color (you can use the same brush to apply the color later on).
Track the the spotty patches
Before applying the deposit-only dye you need to track the patches or spots on your hair. You can sit in front of a mirror or take help from a friend or family member. Just roughly note the patches you need to cover and ask your helper to note them on the back of your head, if any.
While you can easily cover the front, you will need help for the sides and the back.
Start retouching the spots
Once you have identified the spots and patches you need to cover, start applying the hair dye from the bowl using the brush you took earlier.
Carefully rub the color onto the patches, avoiding covering the hair already colored properly. If you cover them, their existing color might also darken, leaving you with patches again.
Another method you can employ is to take a transparent plastic bag and cover it around your head completely. Now ask your friend to mark the spots on the bag using a permanent marker.
Now take the bag off and tear open small holes at the marked spots before wearing the bag again.
Once the bag is back on, your helper can pull out the lightly colored or spotty sections of your hair through the holes you made in the bag.
Now you can simply color these patches without worrying about spilling color over the rest of your hair or ruining your hair color.