At this point in the RP the group should be fairly strong, and we need to power through some of the fights to move the story forward so I'm not as worried about the powers.
For Jester's shadow form, he would only have to dodge effects made by a foe. Random debris shadows, he can move through without worry, the shadow of something used by the Colossus though he would have to deal with because it was used with the intent to harm him.
As for him affecting the Colossus, just imagine that its shadows have a mimicked version of its energies that you have to overcome before the effects on the shadow are translated to the Colossus. The mimicked defenses on the shadow would be slightly easier to overcome, unless your against something that has powers with shadows as well, than the originals since the point of the power is to let you have something easier to target.
Romans 8:31 What, then, shall we say in response to this? If God is for us, who can be against us?
1 Corinthians 13:1-3 If I speak in the tongues of men or of angels, but do not have love, I am only a resounding gong or a clanging cymbal. If I have the gift of prophecy and can fathom all mysteries and all knowledge, and if I have a faith that can move mountains, but do not have love, I am nothing. If I give all I possess to the poor and give over my body to hardship that I may boast, but do not have love, I gain nothing.