What is #efefef

Answer 1

a color value

Answer 2

It's a hexadecimal color value. since the values are the same(efefef), it'll be gray

for more info (explanation of how Hexadecimal Color coding works):
https://stackoverflow.com/questions/22239803/how-does-hexadecimal-color-work

Answer 3

a hex (hexidecimal) color value
its in the pattern of

#rrggbbq

Answer 4

what the value