The colors you select for your poster can come from different sources:
Color Wheel
A color wheel shows relationships between colors. You can use it to select colors that are harmonious and look good together.
Still not sure what colors will work well together? The Canva Color Wheel can show you which colors go together. Two other options are Coolors and Adobe Color.
Your Images
You can also pull colors from an important image and use them in the rest of your poster. This is usually done by using something called an eyedropper tool.
The specific steps vary from program to program, but generally speaking it works like this:
Here are more detailed instructions on the eyedropper tool in PowerPoint. If you are using another program, try doing a Google search for the name of the program and "eydropper tool" or "color picker" for instructions.