I only have mild synesthesia. I don't see colors of things but non colored things indeed have color. For me it's mostly annoying. On IRC someone told me he designed all chatters to have their own colors on his end, and that I was green. Even if I couldn't see it it irked me since I am NOT a green type of person!

Also I have this "piano" app where all keys had different colors, the wrong colors of course...

For a while I rented (before smartphones) a disability friendly PDA. It had a color for every weekday. The WRONG colors of course.

I have a hard time with other people's ideas of colors. Daniel Tammet is "Born on a blue day". I think his was Thursday. That would be a brown day for me, I have two shades of blue in my week, but certainly not his day! Also, for me it's like a normal thing Tuesday is yellow (with a touch of lime), I can't for my life imagine it being different for someone else! LOL.