To understand light-colored residue under the oil cap, compare it under normal, safe conditions: cold versus warm, idle versus steady speed, and light versus higher load. Note whether it is only on the cap or also present on the dipstick and coolant reservoir. Change one condition at a time so the result stays useful.
Do not create a risky test just to reproduce it. Consider recent short trips and ambient temperature; a small cap deposit alone does not prove a head-gasket failure.
Quick educational note
For information, not a diagnosis or repair instruction. Vehicle-specific checks are still required. Consult the manufacturer's guidance and a qualified technician.
How to use this information
This article explains general principles and does not establish what is wrong with your vehicle. Read the content limitations.