The colors you picked for the black cat are perfect and the highlights give it depth and 3 dimensional rendition so it looks like a substantive form. The black cat dominates the picture and is the first thing the viewer sees and then other forms draw the eye to the rest of the canvas. There is a touch of visual tension due to the diagonal placing of the lines of the window sill and the cat bed and the cats placement. By including the darker plants outside the window it balances your blacks and your whites so that neither drown out the other.
Being your first attempt at painting animals you did a very good job. Its really quite difficult as you may have discovered because so much depends on the texture and highlights of fur. A simple photograph can lose a great deal of detail that the human eye would pick up by virtue of being able to discern minute details. This goes for any solid colored fur. Its the same issue with the white cat. Your subject matter is quite difficult because of this. But you rendered the subtlety quite well. I hope you do more animal portraiture.