Fill colors in Excel 2010 and 2013 usually fill each cell with a single solid color, no matter how large the cell in question. To add two background colors to one cell, use the Two Colors fill effect to create a gradient.
Select the cell you want to fill, right-click the cell and choose Format Cells.
Open the Fill tab and click Fill Effects.
Choose Two Colors, and then pick the two colors you want to use with the Color 1 and Color 2 selectors.
Pick a shading style and choose one of the variants.
Click OK on both open windows to set the new fill colors.