At many of the dances now the "formal" is what you would wear to prom. I have been to many a military dance where the younger girls wear the short dresses with halter/spaghetti tops.
Your best bet is a deep color, navy, maroon, black, all Marine Balls are in the fall so mine are dark colors
In the spring I guess any color goes, what color is his uniform? Gold looks really nice against the Army Green. Red looks good with the Marine Dress blues.
Mine are:
long black sheath dress with a maroon lace overlay, short sleeves
a long black velvet with puffy taffeta sleeves and bows,
a maroon velvet fitted long skirt with a taffeta/satin porttrait colored top, 3/4 length sleeves
Typical navy blue prom dress, long, with pearl halter/spaghetti straps
Black full skirt with white lace bodice, long sleeve, satin
Fitted deep purple velvet with a chiffon train attached to the shoulders, sleeveless but covers the shoulders. Train goes only to the end tip of the hem. Pretty much backless.
Shoes: something you can walk in. Black patten leather works with most or a white sandal.
Get a pedi if you wear sandals
Jewely, if the dress has pearls go with pearls, if the dress is a sparkly one then go with diamonds, Claire's or Fancy Q has a ton of prom wear out now.
Shop:
Bridal shops
high end thrift stores--2 of mine cost less than $20
check the mall but Belk's doesn't sell prom dresses anymore.
Macy's has had some really pretty ones in the past
Not places like Deb-these tend to be cheap looking, pretty if you are16 not 32.
Have a great time