renderCell might not be necessary

[oliverjam]

Extracting part of your render into a function can be useful if you've got a huge render method that's gotten confusing, but in this case your renderCell() function just returns a React component. Since a component is effectively already a function you might as well skip the middleman and use it directly:

It's also probably more readable to extract a big render into an actual component rather than a normal function, since it'll fit in better with the rest of the JSX:

const CellList = ({ i } ) => (
<Cell
  className={`cell${i}`}
  value={this.state.cells[i]}
  onClick={() => this.handleClick(i)}
  style = {{backgroundColor: this.state.colours[i]}}
/>
)

This whole thing would probably be solved by using array.map((value, i) => <Cell ... />) though since it would make your render method a lot smaller.