Hi RozettaRose! I feel like I can help chime in here.
Most of the time, concept art and splash art are two different skill sets. Concept art is mainly about problem-solving for a design, and the skillset is focused mainly on coming up with fresh new ideas, rapid iteration (to solve the problems as set by gameplay, narrative, etc), and communicating through visuals. Actually, 95% of concept work looks really rough and ugly! Again, the emphasis is on researching and problem-solving.
Splash art is focused around telling a story through visuals, capturing the moment of the splash art that best suits that character's personality and background, and really perfecting the craftsmanship of lighting, mood, colors, and all that other fun stuff. It has to communicate that character's core fantasy in a single frame, which is really hard to do considering everything that goes into that character!
So, as you can see, they are actually two very different focuses, although there is an overlap in that both roles require a knowledge of art fundamentals as the key objective is communicating to other teams and to the audience through art and visuals. The palette is basically the same, but is used to different end goals. That said, at Riot I've definitely heard of concept artists able to help out on splash a little especially if the artist is super passionate about a certain character, and vice-versa. However, this isn't the norm, as they are essentially two full-time jobs with different specialties.
