The phrase "hallmark style" is correct and usable in written English.
It can be used to describe a distinctive or characteristic manner of doing something, often associated with a particular brand or individual.
Example: "The artist's hallmark style is evident in her use of vibrant colors and bold brush strokes."
Alternatives: "signature style" or "distinctive style".
Its hallmark style combines western realism with a Chinese aesthetic.
And like good little soldiers, the clothing stayed in formation and stuck to the hallmark style of the house.
The wry and slightly naughty name is part of the band's hallmark style, which is earnest without being innocent, and funny without being ironic.
A brilliant novel of the family as tug-of-war, recounted in her hallmark style: repartee we associate today with the plays of Harold Pinter.
What brings Miriam and Ronnee together isn't love, Hallmark style; it's a longing for wholeness, for an integrity that neither woman knows until she sees the other's face.
Until recently, Dr. Waters said, archaeologists had probably overlooked earlier artifacts because the Clovis points are so distinctive and, in contrast, the pre-Clovis material has no hallmark style calling attention to itself.
The two men are seated in an office somewhere; "Best Wishes" appears in Hallmark-style print above the signature, which seems to have been supplied by an autopen.
Right now the stock market is cold and dead, ruling out an IPO, and likely buyers - such as Hallmark-style greeting card companies - aren't looking at the recession with any glee.
Set in 1965 in a small Southern California town, "That's What I Am" is a sentimental Hallmark-style movie (part after-school special, part "Wonder Years" offshoot) whose narrator, Andy Nichol (voiced by an uncredited actor who sounds like Greg Kinnear), looks back on his middle-school experience.
Waters's wisdom repackaged as a Hallmark-style graduation present: That's what it means to be an insider.
The writers of the Platinum Era were being replaced by a more homogenized and Hallmark card style of lyric writing that many shows used in order to be more digestible.
