How to calculate a planned contrast for a mixed model ANOVA?

We are trying to understand how to calculate a planned contrast for a mixed models ANOVA with SPSS syntax. We have two factors one within (IMI) and one between (color), both with two levels. And one dependent variable (SCD). Each line in the database is a subject and one within observation. The goal is to compare one group to the other three (weights 1/3 1/3 -1 1/3).
We have found a way to do it, and we would like to check if it’s the appropriate way to do it:
mixed SCD by IMI color
/fixed = IMI*color
/repeated = color | subject(id) covtype(diag.)
/test = 'contraste' IMI*color 1/3 1/3 -1 1/3.
Also we would like to know if the covtype is the apropriate one.
This is for a pre-registration, any suggestions to improve this syntax are more than welcome.
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