Incidentally – and this is a complete aside – pop-out is how psychologists tell whether people have genuine synaesthesia. Synaesthesia is when people see numbers, or words, as colours. Or they might experience sounds as smells, or whatever.
Let’s say I ask you to tell me the pattern of numeral “2“s in this picture. It’s not impossible, but it’ll take you a few seconds. But a synaesthesiac who sees “2“s as grey and “5“s and pink will see something more like this…