Maybe he considers Yuri's paler hair color and skin color compared to Mari, and their red outfits (albeit Yuri wears more red than Mari).

But to me, such associations are connected to the theory that Yuri might be somehow connected to Mari's younger sibling / cousin (if she ever has had one), possibly being a distant relative in the past. Due to Mari presumably have a bond with her sibling, and after the sibling died, she realized that Yuri has several similarities with her dead sibling, both in appearance and personality, and the two became bonded together as if they are siblings to each other, with Yuri becoming Mari's "lesser" guardian angel.

If you compare Mari's art and Yuri's art, you will know what I mean.

Plus, their names also end with the kana syllable "ri", and both of their names are names of white flowers. If you wonder why, here's my explanation:

茉莉 (Mari's name), means "jasmine" (Jasmines are genrally white in color)

百合 (Yuri's name), means "lily" (Lilies are also generally white in color, albeit there are colored variants too).

Also, their surnames also contains parts that refer to the names of real people: Mari's surname "Okazaki" is an actual surname found in many notable people's (and even fictional characters') names, and Yuri's surname "Saigyouji" contains references to the real-life poet Saigyou, who lived in the later Heian period and early Kamakura period.

Hope all of these theories and explanations make sense to the character design and concepts of both Mari and Yuri!