The size of the columns already indicates that. I think varying the colors as well would be confusing.

Disagree. It's the whole point of the page, so it isn't excessively redundant if you say it more than one way... I'd just put a single column that's a different color (ie, green) for the best day, maybe with an embedded checkmark or some other indicator for the color blind.

Google "comparison chart" and see how <a href="http://www.google.com/imgres?imgurl=http://printgreener.com/... facets</a> can inform a viewer in a simple way.

I don't think the checkmark for colour blind is needed (the colours are supplementary, not essential).

1. The URL - would be better if it's a dedicated domain name (e.g. whenshouldigo.com).

2. I agree with the parent about the color coding. It might be more clear if you use a red-green spectrum for each day (e.g. red for the evening, when things might be more busy, and green for the morning, when things might be less busy).

The problem with that is that it forces a negative opinion on busy days. What if people like to go where other people are?

Here's my suggestion: ditch the black borders on the columns and add very subtle shadows.

I really dig the idea, by the way.