I don´t see any reason why she should punish you. Art is free for everyone, yours, too. You don´t even NEED to feel guilty or something like that. It´s YOUR art, your decision what to do with it, not hers. If it bothers you that much hiding this from your mother it´s probably better to talk to her openly. Tell her how much art means to you, that it´s a unremoveable part of you. She HAS TO accept it when she wants to accept you.
In any case, deviantart is for people who are 18 I think, but colors is for ever age if that´s a problem for her.

It sounds like you have two main options:

Keeping it a secret is what you've already been doing, so that may be easiest. But, as you said, keeping the secret is causing you stress. You have to be very careful or she'll find out. Maybe you feel guilty and that adds to the stress.

The other option is telling her. This can be done multiple ways. The safest approach may be, when the topic comes up, ask her why she doesn't like Colors or DeviantART. Maybe she feels like they are unsafe. In this case, it would mean proving they are safe to gain her permission. Some parents are overly protective and controlling because they are insecure with themselves and their relationship with you, so this would mean building up your relationship first so she feels more comfortable and less insecure. You can also offer to let her supervise your time on Colors (Colors is a better place to start because DeviantART has more things that may bother her) until she sees it's safe and fun.

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