The ideal way to inspect a bedikah cloth is in natural light – but not directly in the sunlight. The Talmud (Niddah 20b) discusses standing in the sunlight and shading the cloth with your hand to inspect it. Holding up the cloth near a window, or inspecting it outside even if the sunlight is not very bright, is acceptable. With indoor lighting, colors on the bedikah may be affected by the light. Some types of artificial lighting can make a stain look redder than it really is, while others may whiten some stains so that them seem less problematic. Even so, it is permitted to inspect the bedikah (even at night) under artificial lighting conditions, taking into account how the lighting may affect the stain.
Nevertheless, if you are unsure of a stain, and you are inspecting it in less than ideal lighting conditions, you should hold onto it to view it later in better lighting.