Help with a windows forms problem

Question

I have a school project and I need to make a Paint-like program. The main issue Im having right now is the creation of the tool that is the Paint bucket. The teacher told us that there is a function that helps with the detection of color around a pixel but I just cant find it. I understand that there are three steps for this tool. The first one is detecting the color of the pixel at the click but that is no problem. The second step is to detect all the surrounding area from the same color but I dont know how to do this. Thank you for your cooperation if you have any idea of what can I use for this problem.