Perhaps you want to change a yellow wall to a white wall or a yellow chair to a white chair. Whatever the surface the yellow paint is on, you can apply layers of white primer and paint over the yellow to change the colour without any yellow seeping through. Use this method for painting over darker colours, too, although additional coats of paint might be required to fully cover old colours.
Fill one of the paint roller trays with primer. Roll on one coat of the white primer over the yellow surface. Allow it to fully dry before proceeding -- follow the manufacturer's instructions. If the yellow colour is bleeding through a lot, apply a second coat of white primer and allow to dry.
Get the second paint roller tray and fill it with the white paint. Use the second roller or brush and apply a coat of white paint to the surface. Allow sufficient time for the paint to dry per manufacturer instructions.
Inspect the surface for any yellow spots or areas of white where yellow can be seen underneath. Apply a second coat of white paint and allow to dry.
Apply a third coat of white paint if needed to fully block out any yellow colour still seeping through.
Use a brush for smaller areas and a paint roller for larger surfaces such as walls. Choose the appropriate paint for the job. For outdoor painting, choose house paints that are resistant to the elements. For small indoor items, oil-based paints can be useful, although you must be especially careful to ventilate the area.
Paint in a well-ventilated area to prevent fume inhalation.