Capitalizing the first word of an independent clause after a colon is optional.
MLA style and Chicago style use a lowercase letter to begin an independent clause following a colon; APA style uses a capital letter. Check with your instructor or with the style guide of your discipline, and be consistent throughout your paper.
Use lowercase after a colon to introduce a list or an appositive.
Exercises:
Capitalization 1
Capitalization 2
Related topic:
Using a colon to introduce a list