Yes, MAGENTA is a Scrabble word!

Letter values in Scrabble US: M 3, A 1, G 2, E 1, N 1, T 1, A 1.

Is it a valid word?

MAGENTA is a valid word in the official Scrabble dictionary (US), the UK Scrabble Dictionary and Words with Friends (ENABLE).

a primary subtractive color for light; a dark purple-red color; the dye for magenta was discovered in 1859, the year of the battle of Magenta
a battle in 1859 in which the French and Sardinian forces under Napoleon III defeated the Austrians under Francis Joseph I

Also: battle of magenta.