Ah, so I had it backwards then, now it makes more sense. Lighter colors indicate higher quantity of orders, darker colors means fewer orders were placed in that range.
Are the orders weighted by size or does 1 buy order equal 1 buy order regardless of if it's an order to buy 1 coin or 1000?
It's weighted by ordersize (so you can distinct small orders and big walls). The color-intensity is calculated with a logarithmic function to give better detail around the best ask/bid price (just like how the orderbook chart is shown on bittrex.com, note the 0.01, 0.1, 1.0, 10.0 scale on the y-axis).
note: max color intensity does not correspond to some absolute maximum orderbooksum, all intensities are relative to the maximum sum measured for that day. Meaning global intensities could become darker during the day when huge order wall's enter the graph.