You're right, the color scale is not the greatest for a precise indication of the sea surface temperature (SST) anomaly. It is meant as an approximation, to give you a general feel for where the departures are strong and where they are only weak. For a more precise figure, go to the Climate Prediction Center web site and click on the El Nino/La Nina item on the left side of the page, and there, if you dig around, you can find a more exact picture of the SST departures. About the strength, we often use the last one month to tell what the current strength is. For August alone, it was near or above 1.5C above normal, depending on which SST data set we use, and this suggests that the event has reached the strong category.