Covering up tile gaps
Grout - you are going to have to trial and error the color since there are about 30 shades of white.   You need to mix the grout - super super small batch, let it dry, make sure color is close enough, then fill the gaps.   You will need to read instructions on how to do this but that is a different question.   Do not just fill the gaps with whatever you find as different coloring will look really really bad.   You can get away with some variance but can't be way off.