You Sky box has RGB output, you just need to a SCART to RGB cable. To be honest on the majority of domestic kit you won't even notice a difference between RGB and Composite inputs as the signal is heavily compressed at the head end. I suspect you have an earth loop problem which can be a pain to find.
Even in the post production studios they use composite inputs on the graded monitors for colour correction.
You can make it yourself if you feel like it, the pin outs for the Sky box are below.
1 Right audio output (±6dB) Right audio output (±6dB)
2 Right audio input Right audio input
3 Left audio output (±6dB) Left audio output (±6dB)
4 Audio ground Audio ground
5 Blue ground Blue ground
6 Left audio input Left audio input
7 Blue output Blue input
8 0-2.5 volts (low) when:
- digibox is in standby (or digibox is out of standby & 'Scart control' setting is "OFF')
- VCR is not in playback or onscreen graphics (OSG) mode
4.5-7.5 volts (High-to-6v) when:
- digibox is out of standby,
- 'scart control' setting is 'on',
- picture format setting is '16:9',
- digibox is outputting a 16:9 programme
or when:
- VCR is in playback or OSG mode
9-12 volts (high-to-12v) when:
- digibox is out of standby,
- scart control setting is on,
- picture format setting is '4:3' (or set to '16:9' but digibox is outputting a 4:3 programme)
or when:
- VCR is in playback or OSG mode. 0-2.5 volts (low) when:
- VCR is not in playback or OSG mode
4.5-7.5 volts (High-to-6v) when:
- VCR is in 16:9 playback or OSG mode
9-12 volts (high-to-12v) when:
- VCR is in 4:3 playback or OSG mode
9 Green ground Green ground
10 N/C N/C
11 Green output Green input
12 N/C N/C
13 Red ground Red ground
14 N/C N/C
15 Red output Red input
16 0 volt (low) when:- pin 8 of the TV scart is low
or when:
- pin 8 of TV scart is high-to-6v / high-to-12v,
- video output setting is 'PAL'
1-3 volts (high) when:
- pin 8 of TV scart is high-to-6v / high-to-12v,
- video output setting is 'RGB',
- digibox is out of standby
or when:
- pin 8 of TV scart is high-to-6v / high-to-12v,
- VCR is in OSG mode 0 volt (low) when:
- pin 8 of VCR scart is low
1-3 volts (high) when:
- pin 8 of VCR scart is high-to-6v or high-to-12v,
- VCR is in OSG mode
17 Composite video output ground Composite video output ground
18 Composite video input ground Composite video input ground
19 Composite video output Composite video output
20 N/C Composite video input
21 Ground Ground