Using SUMPRODUCT across columns when I have blanks

Hi all - I just learned how to use SUMPRODUCT to count the presence of a value across multiple columns, which is useful. However, I've another one that I don't know how to do, and I'm not sure if I need SUMPRODUCT, COUNTIF or some other voodoo.

I have data from a survey I did for work. Respondents checked a box for YES if certain criteria applied to them. Those 4 criteria are each in a column across the top, and a Y appears in the corresponding column if they checked the box. If they didn't check the box, it's blank.

What I want to do is count the instances of someone only checking ONE box out of the 4 - regardless of which one it is.

So:

Name	Yellow	Red	Green	Blue
Joe	Y		Y
Ed			Y
Steve		Y		Y
Jane		Y	Y	Y

I want a formula to tell me that 1 person (Ed) has answered Y in only one instance, and disregard the others because they have more than one yes.

Thanks!

Ah, thank you, @daddylonglegs!