Conversion tracking problem.

Selling mulitple items - how can i track campaign CPA

[samseo]

1. Let us say I am selling T-shirts. RED, BLUE, GREEN and YELLOW.
2. I setup 4 campaigns in my adwords account.
3. I have 4 different conversion pages.
4. I insert coversion codes in to the 4 different thank you pages.
5. My problem is - is someone came to my site by clicking on
RED campaign keyword and ended up buying a BLUE T-shirt - it will
show me a conversion for BLUE - but expense for RED.
6. So my campaign CPA's are not correct. Neither are my keyword costs
nor conversions

How does one solve this?
How can I organiz a campaign to tackle this situation?

[resham5]

what's happening here is a cross-sale.. the solution I can think of here is that you have separate micro-sites for each product if the CPA is that important for you.
The other option is, you manually check your click-cost data and compare it with conversion data and then if a cross sale has happened you count that cost for the other product and not the one for which the click has happened- though this isn't a very feasible option.