1. To create an announcement on a Channel > go to New conversation as shown in Figure 1 below. 
Figure 1

2. Select the format icon 

Figure 2

3. Click on the dropdown option near the New conversation to choose the post type and select Announcement. 

Figure 3

4. After selecting and adding the headline of the Announcement, you can change the colour scheme as well as change the background of your choices. 

Figure 4

Done!