Right way of making tonal corrections:

The Examples are made with respect to Reactive Dyeing:

It is almost imperative to make small changes in the final tone/depth of the shade to meet the requirement of the customer.

Every dyer has his own way of making the dyestuff additions and we recommend the following few safe methods.

Some important factors to be considered before making additions.

How much addition will be necessary to reach the shade? Arrive at a recipe in terms of % of original recipe.
Let us consider that a 5% shade is running on the machine. You assess that a 20% extra dyestuff is required. How do you add this dyestuff?
As usual dissolve the dyestuff, filter it and make it ready for addition.
Now consider that a 20% of 5% original depth is only 5 x 20/100 = 1%.
In the dye bath you have been using for dyeing a depth of 5%, the salt and soda present are two high for a mere1% additional quantity of dyestuff.
This very high concentration at the fixation temperature of 60° or 80°C will easily hydrolyze the 1% dyestuff you are adding. You will not get the required change in shade.
Then what you should do?
Now you require only 1/5 of the original concentration of salt and soda and you should protect the fabric from direct strike off of the dyestuff at the fixation temperature to avoid uneven dyeing.
What you should do?
Drain off 50 to 60% of bath, add fresh water, if necessary add the required amount of some good sequestering agent. Now check the temperature. It will be almost 40°C. Run the machine for 10 to 15 minutes.
Now you inject the additional dyestuff slowly in to the machine.
Run at that temperature for about 1/2 hour. [If it is a cheese yarn dyeing machine, fix the out-in and in-out timings as 5:3 or 6:4, to ensure level dyeing and avoid ring formation.] Check the sample. If the tone and depth are ok, run for another 10 minutes at the same temperature and drain the bath.
If after 1/2 hour the tone and depth are still lighter, increase the temperature to the required level and check for tone and depth.
Please note that there is no need to add further quantity of salt of soda.

What will happen if straightaway the required dyestuff is added in to the same bath without taking all the above precautions?

Unevenness is 60 to 80% possible.
Poor rubbing and washing fastness will result.
Sometimes the depth or tonal changes, will not at all occur due to premature hydrolysation of the additions made to the bath.