"It'll probably be brown, tan, gray, black and white," interior designer Nate Berkus tells PEOPLE of his plans for his son's nursery

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Nate Berkus and Jeremiah Brent have a busy spring ahead of them.

The couple’s daughter Poppy turns 3 on March 24, their design show Nate & Jeremiah by Design returns to TLC April 7 and they’ll celebrate their fourth wedding anniversary on May 3 — all while awaiting the birth of their second child, a baby boy.

So for Poppy’s birthday bash, “It may just be a low-key, the-three-of-us kind of a thing,” Berkus, 46, told PEOPLE at Wednesday night’s Celebrity Edge culinary preview event in New York City.

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Berkus with Poppy and Brent

RELATED VIDEO: TLC’s Nate Berkus and Jeremiah Brent Open Up About Baby No. 2 and the Unique Story Behind Daughter Poppy’s Name

In the meantime, the designer duo has started planning what their son’s nursery will look like.

“We have a lot of things ordered, but I’m Jewish so I have a superstition against it,” admitted Berkus, a Celebrity Edge Travel Ambassador.

“But it’ll be, I think, a masculine version of our daughter’s room. It’ll probably be brown, tan, gray, black and white. That’s our guess.”

While The Things That Matter author has his reservations ahead of baby No. 2’s arrival (which he previously said is happening “any minute”), his daughter is looking forward to becoming a big sister.

“She’s very happy,” Berkus continued Wednesday. “She’s a very sweet girl and she’s very maternal, which is interesting.”

Added the proud papa, “She’s also very clear on what her brother is allowed to play with and what he isn’t, and what she’s willing to help out with. So we’ll see how that pans out!”