I'm trying to get Lydia to equip a certain set of armor, but she will only equip something certain sets. What is the console command to force her to equip something?

Give her the item you want her to wear. Then, open the console. Click on Lydia. Type:

showinventory

There will be a list, look for her "Steel Armor" and write down the number. You can scroll up and down the list with the Page up and Page down keys. Type:

A2C94.removeitem [#####] 1

[#####] is the item number for her steel armor, it is probably 00013952. Now then, find the item number for the object you want her to equip (it should already be in her inventory) Type:

A2C94.equipitem [####]

Again, [####] is the item number. This should work.
EDIT: Do not remove all of her steel armor. Leave her with her boots, or her gauntlets. If you remove all her armor she will reset and put it back on.

Thanks for the help Varus! That worked perfectly.