Generally, all metrics are marked with green, and dimensions are marked in blue.
WHY DOES MY METRIC APPEAR IN BLUE AND NOT IN GREEN?
The color (type) is derived from the selected aggregation rule, not it's datatype.
When this is not set, a column is assumed to be a dimension thus appearing blue.
When picking an aggregation rule, it's shown as a metric and appearing green. While a float value is seldom used as a dimension, it's still technically possible.