When responding to a faulted oil filled transformer emergency call, the first thing you should do is follow your Lockout/Tagout procedures.
The second would be to take oil samples; a general sample and a dissolved gas analysis sample. The typical oil colors are as follows: New Mineral oil = almost clear, but a hint of yellow; Old Mineral Oil = more distinguished yellow; RTEMP = a darkish caramel color with the consistency of molasses; Silicone = clear; Biodegrade (Envirotemp, FR3, etc.)= green or greenish hue. They all have one thing in common! When an oil filled transformer fails/faults within the tank, the oil turns Black! They all have an aroma of burnt paper or a carbon smell. When these oils haven’t been subjected to a fault, they have unique aromas. In the future, use your sense of smell to familiarize yourself with the scents. Side note, PCB’s have a unique scent as well!
For more information about transformer oil samples, refer to this article.