It’s a prom must-have, right up there with the dress and shoes: The guy’s tie must match the girl’s gown. And many teens today use cellphones to aid the color coordination.

Kourtney Ziercher took a picture of her dress in the store to send to her date, Michael George, for her prom last year at Barat Academy in Chesterfield, Mo.

“I told him it was burnt orange, and he got the tie to match,” she said.

“She knew that it was not a typical color, but the tie I got even had a little design on it that matched the design on the dress,” Michael said, referring to a light tiger stripe print on the fabric. “She was really excited about it. The girl’s dress is a big deal. If the outfit doesn’t go perfectly, if the guy isn’t matching, it’s a problem. I made sure it worked.”

But with so many dresses store-bought these days, and with cellphone photos sometimes producing unreliable hues, stores often provide physical swatches to assist in color coordination. David’s Bridal, which expects to sell 110,000 prom dresses this year in 300 stores, sells fabric swatches for $1 with dress purchases.

Sarah LaRue, a senior at Sheldon High School in Eugene, Ore., picked out a dress at the local David’s Bridal and was happy to have the fabric scrap to give her date. “It’s a turquoise blue, and with that dress, you need the exact color to match it,” she said. “If it was even a little bit off, it wouldn’t look right.”

David’s Bridal also has a partnership with Men’s Wearhouse that makes it easy for men to get accessoriesin matching and coordinated hues. Guys can order ties, vests and cummerbunds from Men’s Wearhouse using the same terms David’s Bridal uses to identify the dress colors.

Many couples also plan matching corsages and boutonnieres. “The girls are saying, ‘This is what I’m wearing and I want it to match the flowers,’ ” said Pennylyn Kaine, owner of Blossom & Bee Floral and Event Design in Newfoundland, N.J.