I think it was described here somewhere, but I cant find suitable tutorial fast enough.
It is called masking, better word however would be projecting.
Like the bottom item defines shape with its opaque area and the top one defines also shape and color of light projected on the bottom one.
- add your text field, set it to white, adjust its size and position, consider its size regarding the longest text it is supposed to display
- add your first color image, place and stretch it so, that it covers the whole text
- select the both these layers on the side list of layers. right click selection and pick mask
- it will place them into a group with common name mask, to select the masking image hit the small triangle to roll down the content of this group
- when top image layer in the mask group is selected, click style in right portion and hit the + sign to add image with alternative color I think only 10 styles can be added in WFS
In preview you can then play with separate sets and check how the alternate styles look
