red, brown, bay, neat, harmonious
騂 centers on a reddish-brown horse color (bay) and, by extension, the idea of something neat or harmonious, likely from the pleasing appearance of a well-groomed horse of that color.
騂 combines 馬 (horse) with 辛, which likely contributes the sound. The character specifically refers to a horse with a reddish-brown coat.
A horse (馬) with a coat the color of spicy hot pepper (辛) is a reddish-brown bay horse. The neat, harmonious look comes from the horse's well-kept appearance.
For セイ, picture a bay horse saying 'say' (セイ) as it neighs neatly: the horse's 'say' cues セイ.