Grumpy_Mike: I have no idea what you mean by this.
I have literally no idea which parts I need to connect an appropriate adapter with my circuit. (I mean what kind of cables, ports if possible with links) Also Im kinda confused of the circuit of the external power supply, I made a raw version of what Im thinking about :
Grumpy_Mike: You do know you will only be able to get 7 colours from those RGB LEDs with that circuit don't you?
Yup, I do. @Gerry48
Gerry48: How much current dose the MCP23017 supply?
all combined outputs of one MCP23017 cannot exceed 150mA sink current and one port can sorce or sink a max of 25ma the MCP23017 has 16 Ports
btw I can't thank you all enough for helping me.