A gravid spot is a darkly colored spot on female livebearers, situated just behind the anal fin. It grows larger during pregnancy, getting larger the closer a fish is to giving birth.

What color is the gravid spot?

yellowish colour

Gravid spot of G.
The yellowish colour of the yolk/eggs can be seen when observed under dissecting microscope.

How long is a guppy pregnant before giving birth?

21–30 days

The gestation period of a guppy is typically 21–30 days, but can vary considerably. The area where a pregnant guppy’s abdomen meets the tail is sometimes called the “gravid patch”, or “gravid spot”.