Fohat.digs' post could be misinterpreted. Dupli-color is a brand that has many different types of paint. They make something called "Vinyl Dye", yes. Vinyl dye will seep into the pores of the plastic and change its colour, not be a layer on top of the plastic. Best choice, if you can find it. Vinyl dye is the only option that retains the grainy texture of the plastic. However, you can only dye something in a darker hue than it was before.
If you can't find Vinyl Dye, then use Krylon Fusion without a primer. It was made for plastics.
You could get a very good finish with many hobby spray paints, but it will be more work. Gloss, matte or semi-gloss is up to your preference, it does not matter that much. If you use hobby spray paint, then you should first paint it with a "primer" of the same brand and series. A primer can be better at several things that a paint is not: It covers blemishes very well, it adheres well (there are different primers for different types of surfaces), it can be sanded, most primers are matte grey so make imperfections stand out more, etc.
You could also top off with a "clear coat". If you have painted a matte base coat, then you can make it glossy with a gloss clear coat and vice versa. Clear coat makes the surface more durable.
I recommend that you not only put masking tape on parts that you don't want painted, but that you disassemble the keyboard so that you only paint the case.
Always paint outdoors or in a well-ventilated garage or similar. Paint can create dust, that settles everywhere in the room. Fumes are not fun.
Use plenty of used newspapers, plastic bags or something else underneath the thing you are painting.
You should also prop it up on something or to let it hang from something because if it lies flat on newspaper, the paint will run and stick to it. Paint usually does not stick to plastic bags or plastic tape, though.
