Black (Ancient Egyptian name "kem") : It the color of the fertile silt left by the Nile during its annual flooding giving rise to the Ancient Egyptian name for the country - "kemet" (black land). Black symbolizes fertility, new life/beginnings and resurrection.
The gods were typically represented with gold skin with an exception of Osiris who is depicted with black skin symbolizing fertility, regeneration, and the underworld where the sun was said to regenerate every night.

Yellow/Gold (Ancient Egyptian name "khenet" and "kenit"/"nebu" or "nebw") : It represented the color of women's skin, as well as the skin of people who lived near the Mediterranean. Gold also represented the skin of the gods and the sun itself.
Hence it was used for anything which was considered eternal or indestructible and perfect. The ancient Egyptians made numerous statues of the gods that were either made of gold or covered with gold leaf.