Pillows are the least expensive way to change how a pool deck looks and the fastest way to make a lounger genuinely comfortable. All of these use outdoor rated, fade resistant fabric with fill that drains and dries rather than holding water.
Odd numbers look more natural than even ones on a long sofa. Vary the sizes rather than lining up identical squares. If your furniture is neutral, pillows are where the colour should come from, and because they are inexpensive relative to cushions you can change the palette every couple of seasons without regret.
Headrest pillows are matched to specific chaise and chair frames, so confirm your model before ordering. Throw and bolster pillows are universal.