Posting an answer because a comment would be too long.
Neovim doesn't allow you to set
t_Co. It detects your terminal's color capabilities using unibilium and sets
t_Co accordingly. The color difference you're seeing is because Vim was using an 8 or 16 color palette.
colortest.vim uses named colors.
brown in a 256 color palette will use the color index
130 while <16 color palette will use
yellow in the base palette.
If you want to go back to the color scheme you had before, you will need to change the highlight colors to use numeric values less than 16. But, you may want to just find their 256 color index equivalents and set your terminal's base palette back to colors that match their names: black, red, green, yellow, blue, magenta, cyan, and white. Programs expect these colors to somewhat match the name so they can at least try to ensure that text is legible on colored backgrounds.
If you adjust your theme to use the 256 palette, you could set Vim's
t_Co to 256 for consistency with Neovim.
There are a couple sites that can help with color scheming if you want to go that route:
- terminal.sexy - Helps preview the base 16 color palette. It has a lot of existing themes and can export your colors for specific terminals.
- vivify - Create and preview vim color schemes. This also has a lot of themes. You can customize them by clicking on text to set syntax group colors.
- base16 - Pre-made color schemes for various editors and terminals.
Your color scheme looks fairly close to the base16 "railscast" colors, which you could use as a starting point for customization: