Hi,
In short, field can be Formula OR Linked.
But this is typical case and the solution is to create Automation
in fact, it’s very easy. 
You can use trigger ‘When record matches condition” (Type not empty, Color not empty, Products is empty) or “When record updated” (in Case when record that already linked, can be changed, like other color, and you want it to be re-linked)

Then second step - Update record.
Note - when you have combination, absent in Products table, and you trying to put it into Linked field, it will create new Record in Products and link to it. But if your Products primary field is formula, record cannot be auto-created.

I’ve set up Alexey’s ‘when record matches condition’ solution here for you to check out, and if it works for you do mark his reply as the solution!

Hey ​@Tiago Cabrial,

Adding to Alexey’s answer you might actually want to have two identical automations, but with different triggers. Once for when “record matches conditions” to get executed when Item and ir Color used to be blank. Another one for when “record gets updated” watching the Item and Color fields, in case there is any change to the single select options.

Mike, Consultant @ Automatic Nation

As ​@Mike_AutomaticN said, you can create 2 automations, and I would recommend to start from these simple examples.
Later, when you have more experience and easily setup simple automations, you can move to next level and combine both into one triggered by condition ‘link not OK’

But it’s important to test (even when you changed step 3, for example, you should better start to re-test starting from step 1), and avoid accidentally producing eternal loop of automation runs