Concat rows according to the category

Question

I need to write a code which can concat all the sentences present in a dataframe according to the category.

For example:

Colour	sentences
Red	roses are red
Blue	sky is blue
Red	Red is my favourite colour
Blue	Oceans are blue

Expected output:

Colour	sentences
Red	roses are red Red is my favourite colour
Blue	sky is blue Oceans are blue