Change Color of Solids

Menu: Layer -> Solid Settings

Create a Gradient Mask

Create a Solid. Right click within layers browser then select New -> Solid. Make it black.
Right click the new solid, select Mask -> New Mask. Make sure the blend mode is Add:
Adjust and transform the mask.
Select the layer you want to mask, and under Track Matte, from the dropdown select your solid mask layer from the previous steps: