If you fill an object with a gradient, then apply a color to that object, the color replaces the gradient. The object is then filled with a solid color.
The correct procedure is to create a gradient with color, then apply that colorized gradient to the object, replacing the black-and-white gradient.
At first, it might seem that applying a color to a gradient-filled object should colorize the gradient in the object. However, a gradient can have up to 4 colors in its definition: the beginning color, the ending color and one or two intermediate transition colors. If you try to apply a color to a gradient-filled object, there is no way to specify which of these gradient colors to substitute for the new color. This is why it is necessary to create new gradients with colors in them.