I'm sure someone from Z88 will be along with a more authoritative explanation but I imagine they are enjoying a very much deserved Christmas/New Year break so it might be a while. In the meantime, here's what I think...
Your strip will normally have a colour assigned before you raise the fader. This may be the last colour you had it set to or it may be the default colour (usually white). This is the colour that will apply when the fader first starts to move - the new colour doesn't apply until the fader reaches the trigger level. See the manual here. You can try adjusting the trigger level as described in the manual - this will snap to the new colour when the fader reaches the trigger.
If you want to fade from the old colour to the new one then you need to modify the fader controls, again see the manual here.
As an alternative, at the bottom of the same page of the manual you will find a tip about setting the playback button to "Go" and using it to prep the colour before raising the fader.
If you are busking then you might want to consider creating colour mixing faders rather than recording specific colours on individual playbacks. This is mentioned under "fader controls" but I think there is a longer explanation somewhere. Essentially you create three playbacks one for Red, one for Green and one for Blue and use these to mix your colours live. The trick is to make sure that the required colour is the only one tagged when recording each playback.