It is unlikely that anything terrible would happen. And if there were negative effects, you probably wouldn't see it in a first generation.
If you have your own established line with which you are happy enough & have all the components you are looking for, then it might bring as many negatives as positives introducing new blood.
It sound like this is not the case. If I were you I'd just get on with it & start making your line. The most important thing is to cull ruthlessly. And regarding egg colour.......you probably wouldn't go far wrong if you just never hatch from eggs which would be less than a 4 in full lay when egg colour is usually at its lightest (unless you can track the chicks & restrict their influence on future generations). Try to use males which hatched out of eggs from the darkest egg layer. Sometimes it could be a balance between egg colour & type, but egg colour not below 4 on the chart. Feather colour possibly being one of the least important traits as it is usually easy to sort.