After one spin, you have landed in one of the colours. So, the possible outcomes are:
You landed on a red sector: In this case, you have -1 points. There are 2 red sectors over 7 total sectors, so this happens with probability 2/7.
You landed on a purple sector: In this case, you have 0 points. There are 2 purple sectors over 7 total sectors, so this happens with probability 2/7.
You landed on a yellow sector: In this case, you have 1 point. There are 2 yellow sectors over 7 total sectors, so this happens with probability 2/7.
You landed on a blue sector: In this case, you have 3 points. There is 1 blue sector over 7 total sectors, so this happens with probability 1/7.