To lighten or darken a color do the following: Use RGB and keep the selections in the same proportion. So if R:100, G:100, B:50 is chosen initially, which is a brass color, to darken, lower the values as with R:50, G:50, B:20 and to lighten increase them as with R:200, G:200, B:100. Note: When changing values it is recommended to use only even numbers as it agrees with the math for bit designations of color.
Color Charts
The following sites should make your choice of Background and Text colors easier.