https://www.legislation.gov.uk/ukpga/2006/38/section/36
(1)A person is guilty of an offence if—
(b) he modifies an imitation firearm so that it becomes a realistic imitation firearm;
https://www.legislation.gov.uk/ukpga/2006/38/section/37
(1) It shall be a defence for a person charged with an offence under section 36 in respect of any conduct to show that the conduct was for the purpose only of making the imitation firearm in question available for one or more of the purposes specified in subsection (2).
http://www.legislation.gov.uk/uksi/2007/2606/regulation/3/made
Defences to an offence under section 36 of the 2006 Act or under paragraph 4 of Schedule 2 to that Act
3.—(1) It shall be a defence in proceedings for an offence under section 36 of the 2006 Act or under paragraph 4 of Schedule 2 to that Act for the person charged with the offence to show that his conduct was for the purpose only of making the imitation firearm in question available for one or more of the purposes specified in paragraph (2).
(2) Those purposes are—
(a) the organisation and holding of permitted activities for which public liability insurance is held in relation to liabilities to third parties arising from or in connection with the organisation and holding of those activities;
https://www.gov.uk/government/publications/the-violent-crime-reduction-act-2006-commencement-no-3-order-2007-firearms-measures
The regulations provide for two new defences. The first is for the organisation and holding of airsoft skirmishing. This is defined by reference to “permitted activities” and the defence applies only where third party liability insurance is held in respect of the activities.
tl;dr version - you can spray it any colour you like as long as you can show (note, show, not just assert) that the purpose is to play airsoft at an insured site.
However, if you're a 12 year old wanting to spray your dayglo orange BBshootaz.com gat so that you shit up your mates, gonnae no dae that.