Color-treated hair can be thermally straightened and curled if it has not been overly damaged during the coloring process. Flat irons can get extremely hot and cause damage on their own even when a color treatment is not present. Safe temperature settings and protective sprays and conditioners are recommended to prevent flat-iron damage.
Damage
Flat irons should never be used on severely-damaged hair. Applying heat to damaged hair can result in breakage and/or disintegration of the hair.
Analysis
Overly damaged hair appears limp and lifeless when wet and brittle and fuzzy when dry. Breakage is common, and the hair is hard to style. The hair may even begin to feel like something other than hair.
Products
There are two main products that should be used when using a flat iron on dyed hair. A smoothing, protective styling gel or serum should be used before drying the hair. A protective, thermal spray should be applied to dry hair immediately before straightening.
Temperature Settings
Dyed hair cannot withstand as much heat as hair that has not been dyed. Lower temperature settings are recommended for flat ironing color-treated hair.
Considerations
When in doubt, flat-iron a small section to see how the hair will react. Inspect the hair for signs of damage. Hair that splits, singes or breaks when flat ironed should not be thermally styled.