Cant get my head round this

I've been trying to sort this problem out for the last couple of days but am getting nowhere. I've checked the forum but can't find the answer.

I'm selling doors and I've set up some product names - colour and size. Then set up some values for each each group. Then I've added the attributes to each product. So far so good.

What I want to do is is to use the price for the cheaper coloured doors (for each size) as a base for the wooden doors plus 10% or whatever. Am I making sense?

Here's an example:

Colour: white
size: 500x500
Price: 10.00

Colour: Oak
size 500x500
Price: 11.00 - i.e white door price + 10%

What I'd like is for the white colour price to be the default (for all sizes) and then adjust the prices for the other colours.

I've looked at the option of setting the value for the product and then using the price factor & offset option but this only works for one size (as I understand it). I have around 30 different sizes and want differerent prices for each size. Oh yes and another thing - the price variation needs to be a percentage. Not asking for much am I?:D

I hope this makes sense.

Thanks in advance.