Use gradient fills to apply a graduated blend of colors as you would apply any other color. Creating a gradient fill is a good way to create a smooth color gradation across one or more objects. You can save a gradient as a swatch to make it easy to apply the gradient to multiple objects.
if you want to create a single, multicolored object on which colors can flow in different directions use a mesh object.
For a video on using gradients to enhance your drawings, see www.adobe.com/go/lrvid4017_ai.
To see examples of using gradients, see www.adobe.com/go/learn_ai_tutorials_gradients_en and www.adobe.com/go/learn_ai_tutorials_elliptical_gradients_en and www.adobe.com/go/learn_ai_tutorials_depth_en.
To see an example of using gradients and blending modes, see the tutorial at www.adobe.com/go/learn_ai_tutorials_blendingmode_en.