The word scarlet is actually a synonym for the word red; it's not used as often as it used to be, but red is what scarlet is.  Scarlet is a deep, dark, intense red, very vibrant and rich.  So, the scarlet letter A that Hester wears is of that sort of color.  Hawthorne actually describes the letter in more details, as Hester stands in front of the crowd to be mocked and stared at as part of her punishment.  Near the beginning of the first chapter, when Hester steps out of the prison, Hawthorne describes the A thus:

"On the breast of her gown, in fine red cloth, surrounded with an elaborate embroidery and fantastic flourishes of gold thread, appeared the letter A.  It was so artistically done, and with so much fertility and gorgeous luxuriance of fancy..."

From this description, we can ascertain that it was a very fancy letter, sewed on with gold, shiny thread.  It stood out boldly on her dress, and showed great ability in terms of talent at sewing.  Compared to the black-garbed, dim Puritans that she stood in front of, it had to have stood out quite a bit.  The colors-bright red, and shiny gold, would have been a rare sight for many of the Puritan people of that time.  It is even described as fantastic, luxurious, and gorgeous.  I hope that those thoughts helped; good luck!