• Commander’s cupola periscopes: Games Workshop “Chainmail” followed by two coats of Tamiya X-25 “Clear Green” [1]
• Commander’s cupola machine gun: Vallejo Model Color “Black” 70950
• Commander’s cupola searchlight: Games Workshop “Mithril Silver”
• Turret smoke grenade dispenser covers: Vallejo Panzer Aces “Dark Rubber” 306
• Turret periscopes: Games Workshop “Chainmail” followed by two coats of Tamiya X-25 “Clear Green”
• Turret right storage box cover (sculpting to be detailed later): Vallejo Model Color “Desert Yellow” 70977
• Turret right storage box 7.62 mm belt ammo box (detailed later): Xtraxcrylix “British Khaki Drab” XA1816
• Turret identification panel (sculpting to be detailed later): Games Workshop “Blazing Orange”
• Turret aerial mount (not shown painted in picture): Vallejo Panzer Aces “Dark Rubber” 306
• Gun muzzle: Tamiya “NATO black” XF-69
• Gun thermal sleeve: treated with two coats of AK Interactive “Afrika Korps Filter” AK065 to subtly vary the colour compared with the metal barrel.
• Gun thermal sleeve straps and underneath section: Vallejo Model Color “Khaki” 70988
• Tyres: Vallejo Model Color “Black” 70950
• Hull headlights: Games Workshop “Mithril Silver”
• Hull driver’s periscope:  Games Workshop “Chainmail” followed by two coats of Tamiya X-25 “Clear Green”
• Hull fire extinguisher system handles: Humbrol “Scarlet Matt” 60
• Hull rear red light: Humbrol “Bright Red” 19
• Hull rear red reflector: Humbrol “Scarlet Matt” 60
• Hull cables: Vallejo Model Color “Black Grey” 70862
• Hull tool handles: Vallejo Panzer Aces “Old Wood” 310, Vallejo Panzer Aces “New Wood” 311
• Hull tools (spade and pickaxe): Vallejo Model Color “Black Grey” 70862
• Hull tools (crowbar): Vallejo Model Color “Gunmetal Grey” 70863
• Hull tools (front glacis plate): Vallejo Model Color “Black” 70950
• Hull fuel tank straps:  Vallejo Panzer Aces “Canvas” 314
• Hull fuel tank buckles: Xtraxcrylix “British Khaki Drab” XA1816

References

1. Modelling the Challenger 1 and 2 MBT and Variants, Graeme Davidson and Pat Johnston, Osprey Publishing, 2006. (link)