Answer
There’s a good chance that the effect is caused by your custom stroke. If the stroke shape isn’t completely black (because, for example, it was made using CMYK black insted of RGB black), it will show as a slightly lighter variant of your chosen stroke colour.
To solve your problem: expand the shape (Object > Expand; or Object > Expand Appearance…) and give it the correct colour as a fill colour.
Attribution
Source : Link , Question Author : RunLoop , Answer Author : Vincent