RedSea Calcium test - when do you "achieve"?

[iaa12]

Using the RedSea calcium test kit, looking for some clarification. The instructions say to keep adding reagent with the syringe until the end color (blue) "is achieved". Well, the sample turns blue at some point , but within a couple seconds starts to go back purple. Is the end color "achieved" when it first turns blue - regardless of what happens immediately afterwards - or do you "achieve" by adding until it turns blue and stays blue?

 

[JimWelsh]

Stays blue.