SINGLE LETTER

Riddle and answer in verse:

A fiery adjective you should know,

With letters seven and syllables two.

Add a letter and watch it grow,

To become a noun with paler hue.

 

Answer:

 

The difference is "O" so plain to see, 

If you take the time to stop and think. 

A fire is FLAMING, it's a fiery red,

But the FLAMINGO is a paler pink.