May 03, 2015 02:00 PM

Getting out of the car, on his way to meet his little sister for the first time on Saturday, big brother Prince George could not have looked more cute.

Stealing the show (unintentionally, of course) in traditional blue shorts and a blue knitted cardigan, complete with matching long socks, the designer of his adorable look says blue is Kate’s favorite color for him.

“She says he looks really nice in blue,” Amaia Arrieta, owner of Amaia boutique in Chelsea Green tells PEOPLE, adding: “He is blonde with blue eyes and that’s why blue suits him so well – he looked so gorgeous!”

Anwar Hussein/WireImage

Princess Kate who has been a customer at the boutique since George was a small baby, bought the blue cardigan and matching long socks (Kate is a huge fan of the $10 socks and has bought several pairs) back in November when she visited the tiny store on Chelsea Green just before the start of the Australian royal tour. The shorts and shirt combo was also from another of Kate’s favorite kidswear designers, Rachel Riley.

“She is amazing, like any other mom, she talks about how Prince George is growing up and how he is doing. For us, we just treat her like any other client,” says Arrieta, who has been running the independent store with co-founder Segolene Tresca for 10 years.

Having visited Amaia several times, Kate has become quite the regular. “She is really, really lovely, she asks for our names and she is just really thankful and always in a good mood, never in a hurry.  She makes everyone feel very comfortable around her.”

Shopping alone (although the first time she was with her mom) Kate knows exactly what she likes when it comes to her son’s style choices. “Every piece is very traditional, classic. She normally goes for any sort of blue tone because she says he looks really nice in blue,” says the Spanish-born designer whose designs are also worn by Harper Beckham.

Although Kate normally likes to shop in store herself, the heavily pregnant mom had to rely on online shopping last week (although she sent a staffer to pick up the items).

“She bought some new things, cardigans, shirts and tops for George but she hasn’t bought anything for the new baby yet,” reveals the designer, although Kate could not resist looking at the girls’ babywear last time she was in store.

“I think she must be so happy because every mom is so happy to have a girl.”

And as for Kate’s oh-my-days-is-this-for-real hospital look: “I could not believe it — the heels! You normally have water retention and your feet are swollen but she looked stunning, so serene and happy.”

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— Monique Jessen

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