I am making a game based on a grid and filling in certain squares with certain colors according to some rules. It is level-based, and the difficulty of a level is determined by the size of the grid and how many colors you have to fill in. Should the first level be ridiculously easy to allow players to get used to the UI?
Here is a really good example from the creator of Super Mario. He explains that the first level was designed to introduce the player to the base concepts of the game. It's a very good video. https://youtu.be/zRGRJRUWafY There is actually quite a bit of psychology in the design of the first level.
You should also look at Classical Conditioning, aka "Pavlovian Conditioning". https://en.m.wikipedia.org/wiki/Classical_conditioning Essentially, you allow the player to discover what "feels good" and "feels bad". In the video, when the player hits a block they receive a coin. This is a reward; they now know that hitting those blocks gives rewards, so they would like to try it again. And inversely, falling into a hole or into water(if applicable to your game), will result in death, which naturally is a bad thing, thus they now know that they should refrain from doing it again.
Usually you should.
The purpose of the first level(s) is to make the player familiar with the controls, UI and most basic game mechanics. The first challenge for the player to beat should be how to control the game, then to understand the game, and then to actually play it competently. It's great when you can mark each of these steps with a success milestone in form of a complete level.
For further research I recommend the video Tutorials 101 by Extra Credits.
However, keep in mind that game design is more of an art than a science. There are no absolute rules in game development. Sometimes you need to muddle this progression for the sake of immersion and storytelling. And sometimes it can be refreshing for the player to get pushed out of their comfort zone and challenge them from the first second on.
It's highly recommended to have ridiculously easy ( and few ) mechanics in the first level. Make the first few player mechanics easy ( introduced in the first level ) to understand and learn - this generally allows a broader range of people to play.
You can make it challenging, though. As long as the overall mechanics are simple - you can make players work to actually win.
All endless games embrace this concept ( Threes, 2048, Flappy Bird, Temple Rush and so on ). It's vital to make the game rules easy to understand ( especially at the start of the game ), but that by now way means you should have a trivial game/level to play.
I personally don't think it should be "ridiculously" easy because if it is too easy then it would give the impression to the player that the whole game lacks any challenge.
I think it would be best to have a slightly more simple introduction to the game's mechanics so the user finds it easy to pick up the controls, just make sure they can easily pick up how to play and that they will learn how to play, but it should have a little challenge (not too much, but a little) to engage the player more.