Conditioning fields by the value of other fields in the same form

I wonder if I can condition the value of some fields by dynamically watching other fields when filling the same form.

I have one quantity field, for example:

Quantity (Dropdown, values 1 to 25)

Now when the user selects a value, two fields will appear next. Those fields, combined together, must not exceed the selected value of the Quantity field:

Quantity: 21

Red items: 15 - Blue items (set automatically to 6)

If you change the value of the Blue items (not exceeding 21, of course), the Red should increase or decrease according. It must be easy for the front-end user to do the selection (the Red would be initially set equal to Quantity).

How can this be accomplished?