Select the object you want to fill
Choose the Fill color box in the toolbar
Open the Color panel, Swatches panel, or Properties panel
Click a color to apply it as the fill
Use the Eyedropper tool to sample a color from another object
Use the Recolor Artwork option to change multiple fills at once
Double-click the Fill box to open the Color Picker
Enter specific color values if needed
Use the Live Paint Bucket tool for filled regions in complex artwork
Make sure the object has a fill enabled and not just a stroke
Switch the active target from Stroke to Fill before choosing a color
Use the Gradient panel if you want a gradient fill
Use the Appearance panel to add multiple fills to one object