I got my raditor switch changed on the weekend

The mechanic filled up the anti freeze with blue antifreeze after diluting it as neccessary (approx one part to four)

It started off as nice blue - but has not turned to a dark dirty pink - Is this normal and if I need to top up should I use pink or blue top up

Im thinking of selling the car soon anyway so appearance is important here

Get the cooling system drained, flushed, and refilled ASAP with the correct VAG coolant (G12? Check the manual). The murky crap can silt things up. The proper coolant isn't expensive.