The Garden Planner doesn't recommend which plants to grow after your earlier crops because everyone approaches succession planting in different ways so there's no one recommended way to choose what grows after what. Some gardeners like to follow plants with others from the same crop family, and rotate their crops once a year, while others prefer to rotate crops within a single growing season (you should always do this for succession crops if your plants have suffered from soil-borne pests and diseases this year).
The colored background on each plant indicates which crop family it belongs to - green for brassicas (cabbage family plants), yellow for potato and tomato family, blue for legumes, and so on, to make it easy to select an appropriate crop.
To narrow down what to grow next to only suitable crops, click on Show More at the top right of the plant selector to open the plant filters.
You can choose as many filters as you wish from here (for instance plant type, crop rotation family, easy to grow or frost tolerant).
Scroll down to Planting Times and select the month you wish to sow, plant or harvest from the drop-down menus. Only plants that meet your requirements will then be shown in the plant selector.
Click Reset to return to viewing all plants.