Big 4 digit 7 segment display clock

Hey there,

I'm having a little problem with the current project that I'm working on. I'm trying to make a big arduino clock with a 4 digit 7 segment display made out of ledstrips. Every segment consists of 1 piece of 12v ledstrip with 3 RGB LEDs on it. I'm going to use a DC 12V 1A power adapter to power both the arduino uno and the pieces of ledstrip. I know that you can simply control a 12V ledstrip with arduino using a MOSFET. However, usely you connect it to arduino ground and the negative if the power supply, but I am multifexing the display but can't use a digital pin as ground, because it is 12V. For all 7 segments(I'm not using the dot) I have a MOSFET hooked up, but if I lead all of them to arduino Ground I can't multiflex. I came up with the idea to replace the connect from MOSFET to arduino ground with another MOSFET in between. I will do this for all 4 digits of my clock. Now I can use the pin on the arduino ment for the digit as an output instead of input, so my arduino doesn't get fried. However, I don't know wether I should keep the wire that was originaly going from MOSFET 1 to negative of the power adapter the way it is, or that this one should also go true MOSFET 2 and then to negative power adapter. This way I would need 11 MOSFETs, but can I replays them with MOSFET arrays, like 2 ULN2803A, with 1 being a replacement for the 7 MOSFETs for the segments and the second one a replacement for the digit MOSFETs. Is this possible?

Once I have it all working for 1 colour, I would like to be able to change the colour. The way I see it, I would need to duplicate everything 2 times so I would end up with 33 MOSFETs that need to be controlled. That is not possible with the arduino uno. If I want to do this, do I have to get an arduino mega or is there some chip I can use to be able to control it with the uno?
Please keep in mind that I will also need to add buttons, a speaker(or buzzer) and maybe a real time clock, so I cannot use all the 20 available pins on the uno just for the display.

May I already thank you for an answer.

By the way, sorry for any spelling or grammar mistakes, I'm not a native speaker

Are these 'dumb' RGB LEDs, so the strip just has +12 and then R, G, B outputs - with 12V applied, you connect R, G, or B to Gnd and they turn on?

If so, you need TPIC6B595 shift registers to sink current thru the strips.
4 digits x 7 segments x 3 colors = 84 outputs.
I offer a board with 96 outputs, would be great for driving all those segments.
Here it is populated with 6 shift registers, with all 12 it has 96 low driving outputs.
A '328P is also on the board for Arduino functionality. Plug on an FTDI Basic for code downloading.

There are also spots for 2 transistors if you need to sink more than 150mA, and 12 other IO pins for buttons or I2C to an RTC module.

Thank you for replying,

Yes, they are dumb ledstrips. I'm somewhat new to arduino and electronics so I don't really know what I'm doing. Your solution will probably work but I think I need a lot of knowledge to be able to work with it. Your solution also is kind of expensive, and I'm not willing to pay 100$ for a hobby project. My first goal will be to have it running with just 1 colour, which already seems difficult to me. I've also looked arround on the internet and I found someone using a MAX7219 chip, but it was build in into an 8 digit 7 segment module. Is it possible to use a MAX7219 in my project?