Deal with important colors that are not considered important

[alejandrodemiquel]

In some cases, a color has a specific behaviour. However, we don't identify this color as special because it doesn't appear in ALL of the samples. Tasks 532 (pink color doesn't appear in the first training sample) and 748 (blue and pink don't appear in one sample each) are two examples.

Maybe we should be more flexible in deciding which colors are special? For example, allow a color to not appear in one training sample if there are at least 5.