It's not too difficult, just time consuming. If you have the mod mapwriter, you can use the colors it defines for each block. Create a file in the "Stylesheets" folder that's in the Unmined install folder. There should be a file there already called "unmined.default.json" you need to make a new file, I think you can name it anything you wish, as long as it has a .json extension. It's just a text file, best to edit in Notepad++, follow this format for each block type:
The number referred as "AppliesTo" is the block ID number in game. The base color ID is the color you want in hex. There are several hex color pickers on the web. But, if you have Map Writer, it has already picked colors based on the current texture pack, so that's a bonus. Go to the "config" folder of your mod pack, and look for a file called "MapWriterBlockColours.txt" it is formatted like so:
There are a bunch of biome colors and blocktype entries at the beginning you can ignore. The blocks are listed by number, then by NBT data, so any sub blocks of that type have their own color. An asterisk means any subtype of that block ID will have that color. It's just a matter of transferring the information from one file to the other to have all the block types colored according to the current texture pack in Unmined. A spreadsheet like Excel can make this much easier to do. Just replace the ff at the beginning of the hex number in the Map Writer list with a # in the Unmined list for the color to be input correctly. Megasys has mentioned adding Map Writer support in a new version, let's hope he finds time to do that, it would be cool.