Grey Box Testing:

Grey Box testing is technique that uses a combination of Black box and White box testing. Grey Box testing is not Black box testing, because the tester does know some of the internal workings of the software under test.

In Grey box testing, the tester applies a limited number of test cases to the internal workings of the software under test. In the remaining part of the Grey box testing, one take black box approach in applying inputs to the software under test and observing the outputs.

The typical gray box tester is permitted to set up his testing environment,
like seeding a database, and can view the state of the product after their
actions, like performing a SQL query on the database to be certain of the
values of columns. It is used almost exclusively of client-server testers or
others who use a database as a repository of information.